From da60dc53703cbae972033edd78abac7b52ffc7af Mon Sep 17 00:00:00 2001
From: Wez Furlong <wez@wezfurlong.org>
Date: Fri, 27 Mar 2026 08:37:08 +0000
Subject: [PATCH] mailparsing: switch parser input to byte slices

There's a bit of fuss around migrating the `char` nom parsing function
to `tag`, which is required for operating on bytes.  We need to jump
through some hoops in order to be able to show what tag was expected
if there is a mismatch.  That improves the output of our parse error
type to have less redundant information, so overall I'd say that
that is a plus.

A number of 8-bit related tests now fail and some continue to fail
from an earlier commit in this refactoring series.  We can now
start to tackle whether those need to be adjusted to the amended
API surface, or whether we need to add some supporting context
for those, on a case by case basis in follow-up commits.
---
 crates/kumod/src/http_server/inject_v1.rs |   2 +-
 crates/mailparsing/src/header.rs          |  24 +-
 crates/mailparsing/src/mimepart.rs        |   6 +-
 crates/mailparsing/src/nom_utils.rs       |  62 +--
 crates/mailparsing/src/rfc5322_parser.rs  | 519 +++++++++++++---------
 crates/mailparsing/src/strings.rs         |   5 -
 6 files changed, 353 insertions(+), 265 deletions(-)

diff --git a/crates/kumod/src/http_server/inject_v1.rs b/crates/kumod/src/http_server/inject_v1.rs
index 3a3e9d61..775db3e7 100644
--- a/crates/kumod/src/http_server/inject_v1.rs
+++ b/crates/kumod/src/http_server/inject_v1.rs
@@ -713,7 +713,7 @@ impl InjectV1Request {
                     let opts = mailparsing::AttachmentOptions {
                         file_name: a.file_name.clone().map(|s| s.into()),
                         inline: a.content_id.is_some(),
-                        content_id: a.content_id.clone(),
+                        content_id: a.content_id.clone().map(Into::into),
                     };
 
                     let decoded_data;
diff --git a/crates/mailparsing/src/header.rs b/crates/mailparsing/src/header.rs
index 52d7c106..5e19e9b7 100644
--- a/crates/mailparsing/src/header.rs
+++ b/crates/mailparsing/src/header.rs
@@ -184,57 +184,57 @@ impl<'a> Header<'a> {
     }
 
     pub fn as_content_transfer_encoding(&self) -> Result<MimeParameters> {
-        Parser::parse_content_transfer_encoding_header(self.get_raw_value_string()?)
+        Parser::parse_content_transfer_encoding_header(self.get_raw_value())
     }
 
     pub fn as_content_disposition(&self) -> Result<MimeParameters> {
-        Parser::parse_content_transfer_encoding_header(self.get_raw_value_string()?)
+        Parser::parse_content_transfer_encoding_header(self.get_raw_value())
     }
 
     pub fn as_content_type(&self) -> Result<MimeParameters> {
-        Parser::parse_content_type_header(self.get_raw_value_string()?)
+        Parser::parse_content_type_header(self.get_raw_value())
     }
 
     /// Parse the header into a mailbox-list (as defined in
     /// RFC 5322), which is how the `From` and `Resent-From`,
     /// headers are defined.
     pub fn as_mailbox_list(&self) -> Result<MailboxList> {
-        Parser::parse_mailbox_list_header(self.get_raw_value_string()?)
+        Parser::parse_mailbox_list_header(self.get_raw_value())
     }
 
     /// Parse the header into a mailbox (as defined in
     /// RFC 5322), which is how the `Sender` and `Resent-Sender`
     /// headers are defined.
     pub fn as_mailbox(&self) -> Result<Mailbox> {
-        Parser::parse_mailbox_header(self.get_raw_value_string()?)
+        Parser::parse_mailbox_header(self.get_raw_value())
     }
 
     pub fn as_address_list(&self) -> Result<AddressList> {
-        Parser::parse_address_list_header(self.get_raw_value_string()?)
+        Parser::parse_address_list_header(self.get_raw_value())
     }
 
     pub fn as_message_id(&self) -> Result<MessageID> {
-        Parser::parse_msg_id_header(self.get_raw_value_string()?)
+        Parser::parse_msg_id_header(self.get_raw_value())
     }
 
     pub fn as_content_id(&self) -> Result<MessageID> {
-        Parser::parse_content_id_header(self.get_raw_value_string()?)
+        Parser::parse_content_id_header(self.get_raw_value())
     }
 
     pub fn as_message_id_list(&self) -> Result<Vec<MessageID>> {
-        Parser::parse_msg_id_header_list(self.get_raw_value_string()?)
+        Parser::parse_msg_id_header_list(self.get_raw_value())
     }
 
     pub fn as_unstructured(&self) -> Result<BString> {
-        Parser::parse_unstructured_header(self.get_raw_value_string()?)
+        Parser::parse_unstructured_header(self.get_raw_value())
     }
 
     pub fn as_authentication_results(&self) -> Result<AuthenticationResults> {
-        Parser::parse_authentication_results_header(self.get_raw_value_string()?)
+        Parser::parse_authentication_results_header(self.get_raw_value())
     }
 
     pub fn as_arc_authentication_results(&self) -> Result<ARCAuthenticationResults> {
-        Parser::parse_arc_authentication_results_header(self.get_raw_value_string()?)
+        Parser::parse_arc_authentication_results_header(self.get_raw_value())
     }
 
     pub fn as_date(&self) -> Result<DateTime<FixedOffset>> {
diff --git a/crates/mailparsing/src/mimepart.rs b/crates/mailparsing/src/mimepart.rs
index 87676256..8dbdc03f 100644
--- a/crates/mailparsing/src/mimepart.rs
+++ b/crates/mailparsing/src/mimepart.rs
@@ -834,7 +834,7 @@ impl<'a> MimePart<'a> {
             headers.set_content_disposition(cd)?;
 
             if let Some(id) = &opts.content_id {
-                headers.set_content_id(MessageID(id.to_string()))?;
+                headers.set_content_id(MessageID(id.clone()))?;
             }
         }
 
@@ -1142,7 +1142,7 @@ impl<'a> MimePart<'a> {
         if to_fix.contains(MessageConformance::MISSING_MESSAGE_ID_HEADER) {
             if let Some(message_id) = &settings.message_id {
                 msg.headers_mut()
-                    .set_message_id(MessageID(message_id.clone()))?;
+                    .set_message_id(MessageID(message_id.clone().into()))?;
             }
         }
 
@@ -1243,7 +1243,7 @@ pub struct AttachmentOptions {
     #[serde(default)]
     pub inline: bool,
     #[serde(default)]
-    pub content_id: Option<String>,
+    pub content_id: Option<BString>,
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
diff --git a/crates/mailparsing/src/nom_utils.rs b/crates/mailparsing/src/nom_utils.rs
index 64270d61..8d9ef671 100644
--- a/crates/mailparsing/src/nom_utils.rs
+++ b/crates/mailparsing/src/nom_utils.rs
@@ -1,11 +1,12 @@
+use bstr::ByteSlice;
 use nom::error::{ContextError, ErrorKind};
 use nom_locate::LocatedSpan;
 use std::fmt::{Debug, Write};
 
-pub(crate) type Span<'a> = LocatedSpan<&'a str>;
+pub(crate) type Span<'a> = LocatedSpan<&'a [u8]>;
 pub(crate) type IResult<'a, A, B> = nom::IResult<A, B, ParseError<Span<'a>>>;
 
-pub fn make_span(s: &'_ str) -> Span<'_> {
+pub fn make_span(s: &'_ [u8]) -> Span<'_> {
     Span::new(s)
 }
 
@@ -81,20 +82,22 @@ pub fn explain_nom(input: Span, err: nom::Err<ParseError<Span<'_>>>) -> String {
     match err {
         nom::Err::Error(e) => {
             let mut result = String::new();
-            for (i, (span, kind)) in e.errors.iter().enumerate() {
+            let mut lines_shown = vec![];
+
+            for (span, kind) in e.errors.iter() {
                 if input.is_empty() {
                     match kind {
                         ParseErrorKind::Char(c) => {
-                            write!(&mut result, "{i}: expected '{c}', got empty input\n\n")
+                            write!(&mut result, "Error expected '{c}', got empty input\n\n")
                         }
                         ParseErrorKind::Context(s) => {
-                            write!(&mut result, "{i}: in {s}, got empty input\n\n")
+                            write!(&mut result, "Error in {s}, got empty input\n\n")
                         }
                         ParseErrorKind::External { kind, reason } => {
-                            write!(&mut result, "{i}: {reason} {kind:?}, got empty input\n\n")
+                            write!(&mut result, "Error {reason} {kind:?}, got empty input\n\n")
                         }
                         ParseErrorKind::Nom(e) => {
-                            write!(&mut result, "{i}: in {e:?}, got empty input\n\n")
+                            write!(&mut result, "Error in {e:?}, got empty input\n\n")
                         }
                     }
                     .ok();
@@ -102,21 +105,33 @@ pub fn explain_nom(input: Span, err: nom::Err<ParseError<Span<'_>>>) -> String {
                 }
 
                 let line_number = span.location_line();
-                let line = std::str::from_utf8(span.get_line_beginning())
-                    .unwrap_or("<INVALID: line slice is not utf8!>");
+                let input_line = span.get_line_beginning();
                 // Remap \t in particular, because it can render as multiple
                 // columns and defeat the column number calculation provided
                 // by the Span type
-                let line: String = line
-                    .chars()
-                    .map(|c| match c {
+                let mut line = String::new();
+                for (start, end, c) in input_line.char_indices() {
+                    let c = match c {
                         '\t' => '\u{2409}',
                         '\r' => '\u{240d}',
                         '\n' => '\u{240a}',
-                        _ => c,
-                    })
-                    .collect();
+                        c => c,
+                    };
+
+                    if c == std::char::REPLACEMENT_CHARACTER {
+                        let bytes = &input_line[start..end];
+                        for b in bytes.iter() {
+                            line.push_str(&format!("\\x{b:02X}"));
+                        }
+                    } else {
+                        line.push(c);
+                    }
+                }
+
                 let column = span.get_utf8_column();
+
+                lines_shown.push(line_number);
+
                 let mut caret = " ".repeat(column.saturating_sub(1));
                 caret.push('^');
                 for _ in 1..span.fragment().len() {
@@ -128,7 +143,7 @@ pub fn explain_nom(input: Span, err: nom::Err<ParseError<Span<'_>>>) -> String {
                         if let Some(actual) = span.fragment().chars().next() {
                             write!(
                                 &mut result,
-                                "{i}: at line {line_number}:\n\
+                                "Error at line {line_number}:\n\
                                     {line}\n\
                                     {caret}\n\
                                     expected '{expected}', found {actual}\n\n",
@@ -136,7 +151,7 @@ pub fn explain_nom(input: Span, err: nom::Err<ParseError<Span<'_>>>) -> String {
                         } else {
                             write!(
                                 &mut result,
-                                "{i}: at line {line_number}:\n\
+                                "Error at line {line_number}:\n\
                                     {line}\n\
                                     {caret}\n\
                                     expected '{expected}', got end of input\n\n",
@@ -144,17 +159,12 @@ pub fn explain_nom(input: Span, err: nom::Err<ParseError<Span<'_>>>) -> String {
                         }
                     }
                     ParseErrorKind::Context(context) => {
-                        write!(
-                            &mut result,
-                            "{i}: at line {line_number}, in {context}:\n\
-                                {line}\n\
-                                {caret}\n\n",
-                        )
+                        write!(&mut result, "while parsing {context}\n")
                     }
-                    ParseErrorKind::External { kind, reason } => {
+                    ParseErrorKind::External { kind: _, reason } => {
                         write!(
                             &mut result,
-                            "{i}: at line {line_number}, {reason} {kind:?}:\n\
+                            "Error at line {line_number}, {reason}:\n\
                                 {line}\n\
                                 {caret}\n\n",
                         )
@@ -162,7 +172,7 @@ pub fn explain_nom(input: Span, err: nom::Err<ParseError<Span<'_>>>) -> String {
                     ParseErrorKind::Nom(nom_err) => {
                         write!(
                             &mut result,
-                            "{i}: at line {line_number}, in {nom_err:?}:\n\
+                            "Error at line {line_number}, in {nom_err:?}:\n\
                                 {line}\n\
                                 {caret}\n\n",
                         )
diff --git a/crates/mailparsing/src/rfc5322_parser.rs b/crates/mailparsing/src/rfc5322_parser.rs
index 00d6a019..82e16c76 100644
--- a/crates/mailparsing/src/rfc5322_parser.rs
+++ b/crates/mailparsing/src/rfc5322_parser.rs
@@ -4,16 +4,16 @@ use crate::{MailParsingError, Result, SharedString};
 use bstr::{BStr, BString, ByteSlice, ByteVec};
 use charset_normalizer_rs::Encoding;
 use nom::branch::alt;
-use nom::bytes::complete::{tag, take_while, take_while1};
-use nom::character::complete::{char, satisfy};
+use nom::bytes::complete::{take_while, take_while1, take_while_m_n};
 use nom::combinator::{all_consuming, map, opt, recognize};
 use nom::error::context;
 use nom::multi::{many0, many1, separated_list1};
 use nom::sequence::{delimited, preceded, separated_pair, terminated};
-use nom::Parser as _;
+use nom::{Compare, Input, Parser as _};
 use serde::{Deserialize, Serialize};
 use std::collections::BTreeMap;
 use std::fmt::Debug;
+use std::marker::PhantomData;
 
 impl MailParsingError {
     pub fn from_nom(input: Span, err: nom::Err<ParseError<Span<'_>>>) -> Self {
@@ -21,55 +21,119 @@ impl MailParsingError {
     }
 }
 
-fn is_utf8_non_ascii(c: char) -> bool {
-    let c = c as u32;
+/// Like nom::bytes::complete::tag, except that we print what the tag
+/// was expecting if there was an error.
+/// I feel like this should be the default behavior TBH.
+fn tag<E>(tag: &'static str) -> TagParser<E> {
+    TagParser {
+        tag,
+        e: PhantomData,
+    }
+}
+
+/// Struct to support displaying better errors for tag()
+struct TagParser<E> {
+    tag: &'static str,
+    e: PhantomData<E>,
+}
+
+/// All this fuss to show what we expected for the TagParser impl
+impl<I, Error: nom::error::ParseError<I> + nom::error::FromExternalError<I, String>> nom::Parser<I>
+    for TagParser<Error>
+where
+    I: Input + Compare<&'static str> + nom::AsBytes,
+{
+    type Output = I;
+    type Error = Error;
+
+    fn process<OM: nom::OutputMode>(
+        &mut self,
+        i: I,
+    ) -> nom::PResult<OM, I, Self::Output, Self::Error> {
+        use nom::error::ErrorKind;
+        use nom::{CompareResult, Err, Mode};
+
+        let tag_len = self.tag.input_len();
+
+        match i.compare(self.tag) {
+            CompareResult::Ok => Ok((i.take_from(tag_len), OM::Output::bind(|| i.take(tag_len)))),
+            CompareResult::Incomplete => Err(Err::Error(OM::Error::bind(|| {
+                Error::from_external_error(
+                    i,
+                    ErrorKind::Fail,
+                    format!(
+                        "expected \"{}\" but ran out of input",
+                        self.tag.escape_debug()
+                    ),
+                )
+            }))),
+
+            CompareResult::Error => {
+                let available = i.take(i.input_len().min(tag_len));
+                Err(Err::Error(OM::Error::bind(|| {
+                    Error::from_external_error(
+                        i,
+                        ErrorKind::Fail,
+                        format!(
+                            "expected \"{}\" but found {:?}",
+                            self.tag.escape_debug(),
+                            BStr::new(available.as_bytes())
+                        ),
+                    )
+                })))
+            }
+        }
+    }
+}
+
+fn is_utf8_non_ascii(c: u8) -> bool {
     c == 0 || c >= 0x80
 }
 
 // ctl = { '\u{00}'..'\u{1f}' | "\u{7f}" }
-fn is_ctl(c: char) -> bool {
+fn is_ctl(c: u8) -> bool {
     match c {
-        '\u{00}'..='\u{1f}' | '\u{7f}' => true,
+        b'\x00'..=b'\x1f' | b'\x7f' => true,
         _ => false,
     }
 }
 
-fn not_angle(c: char) -> bool {
+fn not_angle(c: u8) -> bool {
     match c {
-        '<' | '>' => false,
+        b'<' | b'>' => false,
         _ => true,
     }
 }
 
 // char = { '\u{01}'..'\u{7f}' }
-fn is_char(c: char) -> bool {
+fn is_char(c: u8) -> bool {
     match c {
-        '\u{01}'..='\u{ff}' => true,
+        0x01..=0xff => true,
         _ => false,
     }
 }
 
-fn is_especial(c: char) -> bool {
+fn is_especial(c: u8) -> bool {
     match c {
-        '(' | ')' | '<' | '>' | '@' | ',' | ';' | ':' | '/' | '[' | ']' | '?' | '.' | '=' => true,
+        b'(' | b')' | b'<' | b'>' | b'@' | b',' | b';' | b':' | b'/' | b'[' | b']' | b'?'
+        | b'.' | b'=' => true,
         _ => false,
     }
 }
 
-fn is_token(c: char) -> bool {
-    is_char(c) && c != ' ' && !is_especial(c) && !is_ctl(c)
+fn is_token(c: u8) -> bool {
+    is_char(c) && c != b' ' && !is_especial(c) && !is_ctl(c)
 }
 
 // vchar = { '\u{21}'..'\u{7e}' | utf8_non_ascii }
-fn is_vchar(c: char) -> bool {
-    let u = c as u32;
-    (0x21..=0x7e).contains(&u) || is_utf8_non_ascii(c)
+fn is_vchar(c: u8) -> bool {
+    (0x21..=0x7e).contains(&c) || is_utf8_non_ascii(c)
 }
 
-fn is_atext(c: char) -> bool {
+fn is_atext(c: u8) -> bool {
     match c {
-        '!' | '#' | '$' | '%' | '&' | '\'' | '*' | '+' | '-' | '/' | '=' | '?' | '^' | '_'
-        | '`' | '{' | '|' | '}' | '~' => true,
+        b'!' | b'#' | b'$' | b'%' | b'&' | b'\'' | b'*' | b'+' | b'-' | b'/' | b'=' | b'?'
+        | b'^' | b'_' | b'`' | b'{' | b'|' | b'}' | b'~' => true,
         c => c.is_ascii_alphanumeric() || is_utf8_non_ascii(c),
     }
 }
@@ -78,64 +142,64 @@ fn atext(input: Span) -> IResult<Span, Span> {
     context("atext", take_while1(is_atext)).parse(input)
 }
 
-fn is_obs_no_ws_ctl(c: char) -> bool {
+fn is_obs_no_ws_ctl(c: u8) -> bool {
     match c {
-        '\u{01}'..='\u{08}' | '\u{0b}'..='\u{0c}' | '\u{0e}'..='\u{1f}' | '\u{7f}' => true,
+        0x01..=0x08 | 0x0b..=0x0c | 0x0e..=0x1f | 0x7f => true,
         _ => false,
     }
 }
 
-fn is_obs_ctext(c: char) -> bool {
+fn is_obs_ctext(c: u8) -> bool {
     is_obs_no_ws_ctl(c)
 }
 
 // ctext = { '\u{21}'..'\u{27}' | '\u{2a}'..'\u{5b}' | '\u{5d}'..'\u{7e}' | obs_ctext | utf8_non_ascii }
-fn is_ctext(c: char) -> bool {
+fn is_ctext(c: u8) -> bool {
     match c {
-        '\u{21}'..='\u{27}' | '\u{2a}'..='\u{5b}' | '\u{5d}'..='\u{7e}' => true,
+        0x21..=0x27 | 0x2a..=0x5b | 0x5d..=0x7e => true,
         c => is_obs_ctext(c) || is_utf8_non_ascii(c),
     }
 }
 
 // dtext = { '\u{21}'..'\u{5a}' | '\u{5e}'..'\u{7e}' | obs_dtext | utf8_non_ascii }
 // obs_dtext = { obs_no_ws_ctl | quoted_pair }
-fn is_dtext(c: char) -> bool {
+fn is_dtext(c: u8) -> bool {
     match c {
-        '\u{21}'..='\u{5a}' | '\u{5e}'..='\u{7e}' => true,
+        0x21..=0x5a | 0x5e..=0x7e => true,
         c => is_obs_no_ws_ctl(c) || is_utf8_non_ascii(c),
     }
 }
 
 // qtext = { "\u{21}" | '\u{23}'..'\u{5b}' | '\u{5d}'..'\u{7e}' | obs_qtext | utf8_non_ascii }
 // obs_qtext = { obs_no_ws_ctl }
-fn is_qtext(c: char) -> bool {
+fn is_qtext(c: u8) -> bool {
     match c {
-        '\u{21}' | '\u{23}'..='\u{5b}' | '\u{5d}'..='\u{7e}' => true,
+        0x21 | 0x23..=0x5b | 0x5d..=0x7e => true,
         c => is_obs_no_ws_ctl(c) || is_utf8_non_ascii(c),
     }
 }
 
-fn is_tspecial(c: char) -> bool {
+fn is_tspecial(c: u8) -> bool {
     match c {
-        '(' | ')' | '<' | '>' | '@' | ',' | ';' | ':' | '\\' | '"' | '/' | '[' | ']' | '?'
-        | '=' => true,
+        b'(' | b')' | b'<' | b'>' | b'@' | b',' | b';' | b':' | b'\\' | b'"' | b'/' | b'['
+        | b']' | b'?' | b'=' => true,
         _ => false,
     }
 }
 
-fn is_attribute_char(c: char) -> bool {
+fn is_attribute_char(c: u8) -> bool {
     match c {
-        ' ' | '*' | '\'' | '%' => false,
+        b' ' | b'*' | b'\'' | b'%' => false,
         _ => is_char(c) && !is_ctl(c) && !is_tspecial(c),
     }
 }
 
 fn wsp(input: Span) -> IResult<Span, Span> {
-    context("wsp", take_while1(|c| c == ' ' || c == '\t')).parse(input)
+    context("wsp", take_while1(|c| c == b' ' || c == b'\t')).parse(input)
 }
 
 fn newline(input: Span) -> IResult<Span, Span> {
-    context("newline", recognize(preceded(opt(char('\r')), char('\n')))).parse(input)
+    context("newline", recognize(preceded(opt(tag("\r")), tag("\n")))).parse(input)
 }
 
 // fws = { ((wsp* ~ "\r"? ~ "\n")* ~ wsp+) | obs_fws }
@@ -163,7 +227,7 @@ fn obs_fws(input: Span) -> IResult<Span, Span> {
 fn mailbox_list(input: Span) -> IResult<Span, MailboxList> {
     let (loc, mailboxes) = context(
         "mailbox_list",
-        alt((separated_list1(char(','), mailbox), obs_mbox_list)),
+        alt((separated_list1(tag(","), mailbox), obs_mbox_list)),
     )
     .parse(input)?;
     Ok((loc, MailboxList(mailboxes)))
@@ -174,11 +238,11 @@ fn obs_mbox_list(input: Span) -> IResult<Span, Vec<Mailbox>> {
     let (loc, entries) = context(
         "obs_mbox_list",
         many1(preceded(
-            many0(preceded(opt(cfws), char(','))),
+            many0(preceded(opt(cfws), tag(","))),
             (
                 mailbox,
                 many0(preceded(
-                    char(','),
+                    tag(","),
                     alt((map(mailbox, Some), map(cfws, |_| None))),
                 )),
             ),
@@ -221,7 +285,7 @@ fn address_list(input: Span) -> IResult<Span, AddressList> {
     context(
         "address_list",
         alt((
-            map(separated_list1(char(','), address), AddressList),
+            map(separated_list1(tag(","), address), AddressList),
             obs_address_list,
         )),
     )
@@ -233,11 +297,11 @@ fn obs_address_list(input: Span) -> IResult<Span, AddressList> {
     let (loc, entries) = context(
         "obs_address_list",
         many1(preceded(
-            many0(preceded(opt(cfws), char(','))),
+            many0(preceded(opt(cfws), tag(","))),
             (
                 address,
                 many0(preceded(
-                    char(','),
+                    tag(","),
                     alt((map(address, Some), map(cfws, |_| None))),
                 )),
             ),
@@ -269,7 +333,7 @@ fn group(input: Span) -> IResult<Span, Address> {
     let (loc, (name, _, group_list, _)) = context(
         "group",
         terminated(
-            (display_name, char(':'), opt(group_list), char(';')),
+            (display_name, tag(":"), opt(group_list), tag(";")),
             opt(cfws),
         ),
     )
@@ -301,7 +365,7 @@ fn obs_group_list(input: Span) -> IResult<Span, MailboxList> {
     context(
         "obs_group_list",
         map(
-            terminated(many1(preceded(opt(cfws), char(','))), opt(cfws)),
+            terminated(many1(preceded(opt(cfws), tag(","))), opt(cfws)),
             |_| MailboxList(vec![]),
         ),
     )
@@ -336,7 +400,7 @@ fn phrase(input: Span) -> IResult<Span, BString> {
                 map(cfws, |_| None),
                 map(encoded_word, Option::Some),
                 map(word, Option::Some),
-                map(char('.'), |_dot| Some(BString::from("."))),
+                map(tag("."), |_dot| Some(BString::from("."))),
             ))),
         ),
     )
@@ -358,7 +422,7 @@ fn angle_addr(input: Span) -> IResult<Span, AddrSpec> {
         alt((
             delimited(
                 opt(cfws),
-                delimited(char('<'), addr_spec, char('>')),
+                delimited(tag("<"), addr_spec, tag(">")),
                 opt(cfws),
             ),
             obs_angle_addr,
@@ -373,7 +437,7 @@ fn obs_angle_addr(input: Span) -> IResult<Span, AddrSpec> {
         "obs_angle_addr",
         delimited(
             opt(cfws),
-            delimited(char('<'), preceded(obs_route, addr_spec), char('>')),
+            delimited(tag("<"), preceded(obs_route, addr_spec), tag(">")),
             opt(cfws),
         ),
     )
@@ -387,12 +451,12 @@ fn obs_route(input: Span) -> IResult<Span, Span> {
         "obs_route",
         recognize(terminated(
             (
-                many0(alt((cfws, recognize(char(','))))),
-                recognize(char('@')),
+                many0(alt((cfws, recognize(tag(","))))),
+                recognize(tag("@")),
                 recognize(domain),
-                many0((char(','), opt(cfws), opt((char('@'), domain)))),
+                many0((tag(","), opt(cfws), opt((tag("@"), domain)))),
             ),
-            char(':'),
+            tag(":"),
         )),
     )
     .parse(input)
@@ -401,7 +465,7 @@ fn obs_route(input: Span) -> IResult<Span, Span> {
 // addr_spec = { local_part ~ "@" ~ domain }
 fn addr_spec(input: Span) -> IResult<Span, AddrSpec> {
     let (loc, (local_part, domain)) =
-        context("addr_spec", separated_pair(local_part, char('@'), domain)).parse(input)?;
+        context("addr_spec", separated_pair(local_part, tag("@"), domain)).parse(input)?;
     Ok((
         loc,
         AddrSpec {
@@ -411,7 +475,7 @@ fn addr_spec(input: Span) -> IResult<Span, AddrSpec> {
     ))
 }
 
-fn parse_with<'a, R, F>(text: &'a str, parser: F) -> Result<R>
+fn parse_with<'a, R, F>(text: &'a [u8], parser: F) -> Result<R>
 where
     F: Fn(Span<'a>) -> IResult<'a, Span<'a>, R>,
 {
@@ -426,7 +490,7 @@ where
 #[test]
 fn test_addr_spec() {
     k9::snapshot!(
-        parse_with("darth.vader@a.galaxy.far.far.away", addr_spec),
+        parse_with("darth.vader@a.galaxy.far.far.away".as_bytes(), addr_spec),
         r#"
 Ok(
     AddrSpec {
@@ -438,7 +502,10 @@ Ok(
     );
 
     k9::snapshot!(
-        parse_with("\"darth.vader\"@a.galaxy.far.far.away", addr_spec),
+        parse_with(
+            "\"darth.vader\"@a.galaxy.far.far.away".as_bytes(),
+            addr_spec
+        ),
         r#"
 Ok(
     AddrSpec {
@@ -450,19 +517,18 @@ Ok(
     );
 
     k9::snapshot!(
-        parse_with("\"darth\".vader@a.galaxy.far.far.away", addr_spec),
+        parse_with(
+            "\"darth\".vader@a.galaxy.far.far.away".as_bytes(),
+            addr_spec
+        ),
         r#"
 Err(
     HeaderParse(
-        "0: at line 1:
+        "Error at line 1, expected "@" but found ".":
 "darth".vader@a.galaxy.far.far.away
        ^___________________________
-expected '@', found .
-
-1: at line 1, in addr_spec:
-"darth".vader@a.galaxy.far.far.away
-^__________________________________
 
+while parsing addr_spec
 ",
     ),
 )
@@ -470,7 +536,7 @@ expected '@', found .
     );
 
     k9::snapshot!(
-        parse_with("a@[127.0.0.1]", addr_spec),
+        parse_with("a@[127.0.0.1]".as_bytes(), addr_spec),
         r#"
 Ok(
     AddrSpec {
@@ -482,7 +548,7 @@ Ok(
     );
 
     k9::snapshot!(
-        parse_with("a@[IPv6::1]", addr_spec),
+        parse_with("a@[IPv6::1]".as_bytes(), addr_spec),
         r#"
 Ok(
     AddrSpec {
@@ -508,11 +574,11 @@ fn word(input: Span) -> IResult<Span, BString> {
 // obs_local_part = { word ~ (dot ~ word)* }
 fn obs_local_part(input: Span) -> IResult<Span, BString> {
     let (loc, (word, dotted_words)) =
-        context("obs_local_part", (word, many0((char('.'), word)))).parse(input)?;
+        context("obs_local_part", (word, many0((tag("."), word)))).parse(input)?;
     let mut result = word;
 
-    for (dot, w) in dotted_words {
-        result.push_char(dot);
+    for (_dot, w) in dotted_words {
+        result.push(b'.');
         result.push_str(&w);
     }
 
@@ -532,11 +598,11 @@ fn domain(input: Span) -> IResult<Span, BString> {
 // obs_domain = { atom ~ ( dot ~ atom)* }
 fn obs_domain(input: Span) -> IResult<Span, BString> {
     let (loc, (atom, dotted_atoms)) =
-        context("obs_domain", (atom, many0((char('.'), atom)))).parse(input)?;
+        context("obs_domain", (atom, many0((tag("."), atom)))).parse(input)?;
     let mut result = atom;
 
-    for (dot, w) in dotted_atoms {
-        result.push_char(dot);
+    for (_dot, w) in dotted_atoms {
+        result.push(b'.');
         result.push_str(&w);
     }
 
@@ -550,12 +616,12 @@ fn domain_literal(input: Span) -> IResult<Span, BString> {
         delimited(
             opt(cfws),
             delimited(
-                char('['),
+                tag("["),
                 (
-                    many0((opt(fws), alt((satisfy(is_dtext), quoted_pair)))),
+                    many0((opt(fws), alt((take_while_m_n(1, 1, is_dtext), quoted_pair)))),
                     opt(fws),
                 ),
-                char(']'),
+                tag("]"),
             ),
             opt(cfws),
         ),
@@ -568,7 +634,7 @@ fn domain_literal(input: Span) -> IResult<Span, BString> {
         if let Some(a) = a {
             result.push_str(&a);
         }
-        result.push_char(b);
+        result.push_str(b);
     }
     if let Some(t) = trailer {
         result.push_str(&t);
@@ -580,7 +646,7 @@ fn domain_literal(input: Span) -> IResult<Span, BString> {
 // dot_atom_text = @{ atext ~ ("." ~ atext)* }
 fn dot_atom_text(input: Span) -> IResult<Span, BString> {
     let (loc, (a, b)) =
-        context("dot_atom_text", (atext, many0(preceded(char('.'), atext)))).parse(input)?;
+        context("dot_atom_text", (atext, many0(preceded(tag("."), atext)))).parse(input)?;
     let mut result: BString = (*a).into();
     for item in b {
         result.push(b'.');
@@ -599,7 +665,7 @@ fn dot_atom(input: Span) -> IResult<Span, BString> {
 #[test]
 fn test_dot_atom() {
     k9::snapshot!(
-        parse_with("hello", dot_atom),
+        parse_with("hello".as_bytes(), dot_atom),
         r#"
 Ok(
     "hello",
@@ -608,7 +674,7 @@ Ok(
     );
 
     k9::snapshot!(
-        parse_with("hello.there", dot_atom),
+        parse_with("hello.there".as_bytes(), dot_atom),
         r#"
 Ok(
     "hello.there",
@@ -617,11 +683,11 @@ Ok(
     );
 
     k9::snapshot!(
-        parse_with("hello.", dot_atom),
+        parse_with("hello.".as_bytes(), dot_atom),
         r#"
 Err(
     HeaderParse(
-        "0: at line 1, in Eof:
+        "Error at line 1, in Eof:
 hello.
      ^
 
@@ -632,7 +698,7 @@ hello.
     );
 
     k9::snapshot!(
-        parse_with("(wat)hello", dot_atom),
+        parse_with("(wat)hello".as_bytes(), dot_atom),
         r#"
 Ok(
     "hello",
@@ -657,7 +723,7 @@ fn cfws(input: Span) -> IResult<Span, Span> {
 fn comment(input: Span) -> IResult<Span, Span> {
     context(
         "comment",
-        recognize((char('('), many0((opt(fws), ccontent)), opt(fws), char(')'))),
+        recognize((tag("("), many0((opt(fws), ccontent)), opt(fws), tag(")"))),
     )
     .parse(input)
 }
@@ -666,17 +732,8 @@ fn comment(input: Span) -> IResult<Span, Span> {
 #[test]
 fn test_comment() {
     k9::snapshot!(
-        parse_with("(wat)", comment),
-        r#"
-Ok(
-    LocatedSpan {
-        offset: 0,
-        line: 1,
-        fragment: "(wat)",
-        extra: (),
-    },
-)
-"#
+        BStr::new(&parse_with("(wat)".as_bytes(), comment).unwrap()),
+        "(wat)"
     );
 }
 
@@ -685,7 +742,7 @@ fn ccontent(input: Span) -> IResult<Span, Span> {
     context(
         "ccontent",
         recognize(alt((
-            recognize(satisfy(is_ctext)),
+            recognize(take_while_m_n(1, 1, is_ctext)),
             recognize(quoted_pair),
             comment,
             recognize(encoded_word),
@@ -694,17 +751,21 @@ fn ccontent(input: Span) -> IResult<Span, Span> {
     .parse(input)
 }
 
-fn is_quoted_pair(c: char) -> bool {
+fn is_quoted_pair(c: u8) -> bool {
     match c {
-        '\u{00}' | '\r' | '\n' | ' ' => true,
+        0x00 | b'\r' | b'\n' | b' ' => true,
         c => is_obs_no_ws_ctl(c) || is_vchar(c),
     }
 }
 
 // quoted_pair = { ( "\\"  ~ (vchar | wsp)) | obs_qp }
 // obs_qp = { "\\" ~ ( "\u{00}" | obs_no_ws_ctl | "\r" | "\n") }
-fn quoted_pair(input: Span) -> IResult<Span, char> {
-    context("quoted_pair", preceded(char('\\'), satisfy(is_quoted_pair))).parse(input)
+fn quoted_pair(input: Span) -> IResult<Span, Span> {
+    context(
+        "quoted_pair",
+        preceded(tag("\\"), take_while_m_n(1, 1, is_quoted_pair)),
+    )
+    .parse(input)
 }
 
 // encoded_word = { "=?" ~ charset ~ ("*" ~ language)? ~ "?" ~ encoding ~ "?" ~ encoded_text ~ "?=" }
@@ -715,10 +776,10 @@ fn encoded_word(input: Span) -> IResult<Span, BString> {
             tag("=?"),
             (
                 charset,
-                opt(preceded(char('*'), language)),
-                char('?'),
+                opt(preceded(tag("*"), language)),
+                tag("?"),
                 encoding,
-                char('?'),
+                tag("?"),
                 encoded_text,
             ),
             tag("?="),
@@ -727,7 +788,7 @@ fn encoded_word(input: Span) -> IResult<Span, BString> {
     .parse(input)?;
 
     let bytes = match *encoding.fragment() {
-        "B" | "b" => data_encoding::BASE64_MIME
+        b"B" | b"b" => data_encoding::BASE64_MIME
             .decode(text.as_bytes())
             .map_err(|err| {
                 make_context_error(
@@ -735,12 +796,12 @@ fn encoded_word(input: Span) -> IResult<Span, BString> {
                     format!("encoded_word: base64 decode failed: {err:#}"),
                 )
             })?,
-        "Q" | "q" => {
+        b"Q" | b"q" => {
             // for rfc2047 header encoding, _ can be used to represent a space
             let munged = text.replace("_", " ");
             // The quoted_printable crate will unhelpfully strip trailing space
             // from the decoded input string, and we must track and restore it
-            let had_trailing_space = munged.ends_with(' ');
+            let had_trailing_space = munged.ends_with_str(" ");
             let mut decoded = quoted_printable::decode(munged, quoted_printable::ParseMode::Robust)
                 .map_err(|err| {
                     make_context_error(
@@ -754,6 +815,7 @@ fn encoded_word(input: Span) -> IResult<Span, BString> {
             decoded
         }
         encoding => {
+            let encoding = BStr::new(encoding);
             return Err(make_context_error(
                 input,
                 format!(
@@ -763,17 +825,27 @@ fn encoded_word(input: Span) -> IResult<Span, BString> {
         }
     };
 
-    let charset = Encoding::by_name(&*charset).ok_or_else(|| {
+    let charset_name = charset.to_str().map_err(|err| {
         make_context_error(
             input,
-            format!("encoded_word: unsupported charset '{charset}'"),
+            format!(
+                "encoded_word: charset {} is not UTF-8: {err}",
+                BStr::new(*charset)
+            ),
+        )
+    })?;
+
+    let charset = Encoding::by_name(&*charset_name).ok_or_else(|| {
+        make_context_error(
+            input,
+            format!("encoded_word: unsupported charset '{charset_name}'"),
         )
     })?;
 
     let decoded = charset.decode_simple(&bytes).map_err(|err| {
         make_context_error(
             input,
-            format!("encoded_word: failed to decode as '{charset}': {err}"),
+            format!("encoded_word: failed to decode as '{charset_name}': {err}"),
         )
     })?;
 
@@ -782,24 +854,24 @@ fn encoded_word(input: Span) -> IResult<Span, BString> {
 
 // charset = @{ (!"*" ~ token)+ }
 fn charset(input: Span) -> IResult<Span, Span> {
-    context("charset", take_while1(|c| c != '*' && is_token(c))).parse(input)
+    context("charset", take_while1(|c| c != b'*' && is_token(c))).parse(input)
 }
 
 // language = @{ token+ }
 fn language(input: Span) -> IResult<Span, Span> {
-    context("language", take_while1(|c| c != '*' && is_token(c))).parse(input)
+    context("language", take_while1(|c| c != b'*' && is_token(c))).parse(input)
 }
 
 // encoding = @{ token+ }
 fn encoding(input: Span) -> IResult<Span, Span> {
-    context("encoding", take_while1(|c| c != '*' && is_token(c))).parse(input)
+    context("encoding", take_while1(|c| c != b'*' && is_token(c))).parse(input)
 }
 
 // encoded_text = @{ (!( " " | "?") ~ vchar)+ }
 fn encoded_text(input: Span) -> IResult<Span, Span> {
     context(
         "encoded_text",
-        take_while1(|c| is_vchar(c) && c != ' ' && c != '?'),
+        take_while1(|c| is_vchar(c) && c != b' ' && c != b'?'),
     )
     .parse(input)
 }
@@ -811,9 +883,9 @@ fn quoted_string(input: Span) -> IResult<Span, BString> {
         delimited(
             opt(cfws),
             delimited(
-                char('"'),
+                tag("\""),
                 (many0((opt(fws), qcontent)), opt(fws)),
-                char('"'),
+                tag("\""),
             ),
             opt(cfws),
         ),
@@ -825,7 +897,7 @@ fn quoted_string(input: Span) -> IResult<Span, BString> {
         if let Some(a) = a {
             result.push_str(&a);
         }
-        result.push_char(b);
+        result.push_str(b);
     }
     if let Some(t) = trailer {
         result.push_str(&t);
@@ -834,8 +906,12 @@ fn quoted_string(input: Span) -> IResult<Span, BString> {
 }
 
 // qcontent = { qtext | quoted_pair }
-fn qcontent(input: Span) -> IResult<Span, char> {
-    context("qcontent", alt((satisfy(is_qtext), quoted_pair))).parse(input)
+fn qcontent(input: Span) -> IResult<Span, Span> {
+    context(
+        "qcontent",
+        alt((take_while_m_n(1, 1, is_qtext), quoted_pair)),
+    )
+    .parse(input)
 }
 
 fn content_id(input: Span) -> IResult<Span, MessageID> {
@@ -852,14 +928,19 @@ fn relaxed_msg_id(input: Span) -> IResult<Span, MessageID> {
     let (loc, id) = context(
         "msg_id",
         delimited(
-            preceded(opt(cfws), char('<')),
-            many0(satisfy(not_angle)),
-            preceded(char('>'), opt(cfws)),
+            preceded(opt(cfws), tag("<")),
+            many0(take_while_m_n(1, 1, not_angle)),
+            preceded(tag(">"), opt(cfws)),
         ),
     )
     .parse(input)?;
 
-    Ok((loc, MessageID(id.into_iter().collect())))
+    let mut result = BString::default();
+    for item in id.into_iter() {
+        result.push_str(*item);
+    }
+
+    Ok((loc, MessageID(result)))
 }
 
 // msg_id_list = { msg_id+ }
@@ -896,14 +977,18 @@ fn strict_msg_id(input: Span) -> IResult<Span, MessageID> {
     let (loc, (left, _, right)) = context(
         "msg_id",
         delimited(
-            preceded(opt(cfws), char('<')),
-            (id_left, char('@'), id_right),
-            preceded(char('>'), opt(cfws)),
+            preceded(opt(cfws), tag("<")),
+            (id_left, tag("@"), id_right),
+            preceded(tag(">"), opt(cfws)),
         ),
     )
     .parse(input)?;
 
-    Ok((loc, MessageID(format!("{left}@{right}"))))
+    let mut result: BString = left.into();
+    result.push_char('@');
+    result.push_str(right);
+
+    Ok((loc, MessageID(result)))
 }
 
 // obs_unstruct = { (( "\r"* ~ "\n"* ~ ((encoded_word | obs_utext)~ "\r"* ~ "\n"*)+) | fws)+ }
@@ -911,7 +996,7 @@ fn unstructured(input: Span) -> IResult<Span, BString> {
     #[derive(Debug)]
     enum Word {
         Encoded(BString),
-        UText(char),
+        UText(BString),
         Fws,
     }
 
@@ -919,13 +1004,13 @@ fn unstructured(input: Span) -> IResult<Span, BString> {
         "unstructured",
         many0(alt((
             preceded(
-                map(take_while(|c| c == '\r' || c == '\n'), |_| Word::Fws),
+                map(take_while(|c| c == b'\r' || c == b'\n'), |_| Word::Fws),
                 terminated(
                     alt((
                         map(encoded_word, Word::Encoded),
-                        map(obs_utext, Word::UText),
+                        map(obs_utext, |s| Word::UText((*s).into())),
                     )),
-                    map(take_while(|c| c == '\r' || c == '\n'), |_| Word::Fws),
+                    map(take_while(|c| c == b'\r' || c == b'\n'), |_| Word::Fws),
                 ),
             ),
             map(fws, |_| Word::Fws),
@@ -959,7 +1044,7 @@ fn unstructured(input: Span) -> IResult<Span, BString> {
                 }
             }
             Word::UText(c) => match processed.last_mut() {
-                Some(ProcessedWord::Text(prior)) => prior.push_char(c),
+                Some(ProcessedWord::Text(prior)) => prior.push_str(c),
                 _ => processed.push(ProcessedWord::Text(c.to_string().into())),
             },
         }
@@ -985,10 +1070,10 @@ fn arc_authentication_results(input: Span) -> IResult<Span, ARCAuthenticationRes
         "arc_authentication_results",
         map(
             (
-                preceded(opt(cfws), char('i')),
-                preceded(opt(cfws), char('=')),
+                preceded(opt(cfws), tag("i")),
+                preceded(opt(cfws), tag("=")),
                 preceded(opt(cfws), nom::character::complete::u8),
-                preceded(opt(cfws), char(';')),
+                preceded(opt(cfws), tag(";")),
                 preceded(opt(cfws), value),
                 opt(preceded(cfws, nom::character::complete::u32)),
                 alt((no_result, many1(resinfo))),
@@ -1028,7 +1113,7 @@ fn authentication_results(input: Span) -> IResult<Span, AuthenticationResults> {
 fn no_result(input: Span) -> IResult<Span, Vec<AuthenticationResult>> {
     context(
         "no_result",
-        map((opt(cfws), char(';'), opt(cfws), tag("none")), |_| vec![]),
+        map((opt(cfws), tag(";"), opt(cfws), tag("none")), |_| vec![]),
     )
     .parse(input)
 }
@@ -1039,7 +1124,7 @@ fn resinfo(input: Span) -> IResult<Span, AuthenticationResult> {
         map(
             (
                 opt(cfws),
-                char(';'),
+                tag(";"),
                 methodspec,
                 opt(preceded(cfws, reasonspec)),
                 opt(many1(propspec)),
@@ -1067,7 +1152,7 @@ fn methodspec(input: Span) -> IResult<Span, (BString, Option<u32>, BString)> {
                 opt(cfws),
                 (keyword, opt(methodversion)),
                 opt(cfws),
-                char('='),
+                tag("="),
                 opt(cfws),
                 keyword,
             ),
@@ -1083,7 +1168,7 @@ fn keyword(input: Span) -> IResult<Span, BString> {
     context(
         "keyword",
         map(
-            take_while1(|c: char| c.is_ascii_alphanumeric() || c == '+' || c == '-'),
+            take_while1(|c: u8| c.is_ascii_alphanumeric() || c == b'+' || c == b'-'),
             |s: Span| (*s).into(),
         ),
     )
@@ -1094,7 +1179,7 @@ fn methodversion(input: Span) -> IResult<Span, u32> {
     context(
         "methodversion",
         preceded(
-            (opt(cfws), char('/'), opt(cfws)),
+            (opt(cfws), tag("/"), opt(cfws)),
             nom::character::complete::u32,
         ),
     )
@@ -1105,7 +1190,7 @@ fn reasonspec(input: Span) -> IResult<Span, BString> {
     context(
         "reason",
         map(
-            (tag("reason"), opt(cfws), char('='), opt(cfws), value),
+            (tag("reason"), opt(cfws), tag("="), opt(cfws), value),
             |(_, _, _, _, value)| value,
         ),
     )
@@ -1120,19 +1205,19 @@ fn propspec(input: Span) -> IResult<Span, (BString, BString)> {
                 opt(cfws),
                 keyword,
                 opt(cfws),
-                char('.'),
+                tag("."),
                 opt(cfws),
                 keyword,
                 opt(cfws),
-                char('='),
+                tag("="),
                 opt(cfws),
                 alt((
-                    map(preceded(char('@'), domain), |d| {
+                    map(preceded(tag("@"), domain), |d| {
                         let mut at_dom = BString::from("@");
                         at_dom.push_str(d);
                         at_dom
                     }),
-                    map(separated_pair(local_part, char('@'), domain), |(u, d)| {
+                    map(separated_pair(local_part, tag("@"), domain), |(u, d)| {
                         let mut result: BString = u.into();
                         result.push(b'@');
                         result.push_str(d);
@@ -1153,16 +1238,16 @@ fn propspec(input: Span) -> IResult<Span, (BString, BString)> {
 }
 
 // obs_utext = @{ "\u{00}" | obs_no_ws_ctl | vchar }
-fn obs_utext(input: Span) -> IResult<Span, char> {
+fn obs_utext(input: Span) -> IResult<Span, Span> {
     context(
         "obs_utext",
-        satisfy(|c| c == '\u{00}' || is_obs_no_ws_ctl(c) || is_vchar(c)),
+        take_while_m_n(1, 1, |c| c == 0x00 || is_obs_no_ws_ctl(c) || is_vchar(c)),
     )
     .parse(input)
 }
 
-fn is_mime_token(c: char) -> bool {
-    is_char(c) && c != ' ' && !is_ctl(c) && !is_tspecial(c)
+fn is_mime_token(c: u8) -> bool {
+    is_char(c) && c != b' ' && !is_ctl(c) && !is_tspecial(c)
 }
 
 // mime_token = { (!(" " | ctl | tspecials) ~ char)+ }
@@ -1182,7 +1267,7 @@ fn content_type(input: Span) -> IResult<Span, MimeParameters> {
             (
                 mime_token,
                 opt(cfws),
-                char('/'),
+                tag("/"),
                 opt(cfws),
                 mime_token,
                 opt(cfws),
@@ -1194,7 +1279,7 @@ fn content_type(input: Span) -> IResult<Span, MimeParameters> {
                     // In the meantime, there are implementations that assume
                     // that the `;` is optional, so we therefore allow them
                     // to be optional here in our implementation
-                    preceded(opt(char(';')), opt(cfws)),
+                    preceded(opt(tag(";")), opt(cfws)),
                     terminated(parameter, opt(cfws)),
                 )),
             ),
@@ -1202,14 +1287,11 @@ fn content_type(input: Span) -> IResult<Span, MimeParameters> {
     )
     .parse(input)?;
 
-    let value = format!("{mime_type}/{mime_subtype}");
-    Ok((
-        loc,
-        MimeParameters {
-            value: value.into(),
-            parameters,
-        },
-    ))
+    let mut value: BString = (*mime_type).into();
+    value.push_char('/');
+    value.push_str(mime_subtype);
+
+    Ok((loc, MimeParameters { value, parameters }))
 }
 
 fn content_transfer_encoding(input: Span) -> IResult<Span, MimeParameters> {
@@ -1228,7 +1310,7 @@ fn content_transfer_encoding(input: Span) -> IResult<Span, MimeParameters> {
                     // In the meantime, there are implementations that assume
                     // that the `;` is optional, so we therefore allow them
                     // to be optional here in our implementation
-                    preceded(opt(char(';')), opt(cfws)),
+                    preceded(opt(tag(";")), opt(cfws)),
                     terminated(parameter, opt(cfws)),
                 )),
             ),
@@ -1268,7 +1350,7 @@ fn param_with_unquoted_rfc2047(input: Span) -> IResult<Span, MimeParameter> {
     context(
         "param_with_unquoted_rfc2047",
         map(
-            (attribute, opt(cfws), char('='), opt(cfws), encoded_word),
+            (attribute, opt(cfws), tag("="), opt(cfws), encoded_word),
             |(name, _, _, _, value)| MimeParameter {
                 name: name.as_bytes().into(),
                 value: value.as_bytes().into(),
@@ -1289,9 +1371,9 @@ fn param_with_quoted_rfc2047(input: Span) -> IResult<Span, MimeParameter> {
             (
                 attribute,
                 opt(cfws),
-                char('='),
+                tag("="),
                 opt(cfws),
-                delimited(char('"'), encoded_word, char('"')),
+                delimited(tag("\""), encoded_word, tag("\"")),
             ),
             |(name, _, _, _, value)| MimeParameter {
                 name: name.as_bytes().into(),
@@ -1313,17 +1395,17 @@ fn extended_param_with_charset(input: Span) -> IResult<Span, MimeParameter> {
             (
                 attribute,
                 opt(section),
-                char('*'),
+                tag("*"),
                 opt(cfws),
-                char('='),
+                tag("="),
                 opt(cfws),
                 opt(mime_charset),
-                char('\''),
+                tag("'"),
                 opt(mime_language),
-                char('\''),
+                tag("'"),
                 map(
                     recognize(many0(alt((ext_octet, take_while1(is_attribute_char))))),
-                    |s: Span| s.to_string(),
+                    |s: Span| (*s).into(),
                 ),
             ),
             |(name, section, _, _, _, _, mime_charset, _, mime_language, _, value)| MimeParameter {
@@ -1332,7 +1414,7 @@ fn extended_param_with_charset(input: Span) -> IResult<Span, MimeParameter> {
                 mime_charset: mime_charset.map(|s| s.as_bytes().into()),
                 mime_language: mime_language.map(|s| s.as_bytes().into()),
                 encoding: MimeParameterEncoding::Rfc2231,
-                value: value.into(),
+                value,
             },
         ),
     )
@@ -1346,15 +1428,15 @@ fn extended_param_no_charset(input: Span) -> IResult<Span, MimeParameter> {
             (
                 attribute,
                 opt(section),
-                opt(char('*')),
+                opt(tag("*")),
                 opt(cfws),
-                char('='),
+                tag("="),
                 opt(cfws),
                 alt((
                     quoted_string,
                     map(
                         recognize(many0(alt((ext_octet, take_while1(is_attribute_char))))),
-                        |s: Span| s.to_string().into(),
+                        |s: Span| (*s).into(),
                     ),
                 )),
             ),
@@ -1378,7 +1460,7 @@ fn extended_param_no_charset(input: Span) -> IResult<Span, MimeParameter> {
 fn mime_charset(input: Span) -> IResult<Span, Span> {
     context(
         "mime_charset",
-        take_while1(|c| is_mime_token(c) && c != '\''),
+        take_while1(|c| is_mime_token(c) && c != b'\''),
     )
     .parse(input)
 }
@@ -1386,7 +1468,7 @@ fn mime_charset(input: Span) -> IResult<Span, Span> {
 fn mime_language(input: Span) -> IResult<Span, Span> {
     context(
         "mime_language",
-        take_while1(|c| is_mime_token(c) && c != '\''),
+        take_while1(|c| is_mime_token(c) && c != b'\''),
     )
     .parse(input)
 }
@@ -1395,9 +1477,8 @@ fn ext_octet(input: Span) -> IResult<Span, Span> {
     context(
         "ext_octet",
         recognize((
-            char('%'),
-            satisfy(|c| c.is_ascii_hexdigit()),
-            satisfy(|c| c.is_ascii_hexdigit()),
+            tag("%"),
+            take_while_m_n(2, 2, |b: u8| b.is_ascii_hexdigit()),
         )),
     )
     .parse(input)
@@ -1405,11 +1486,7 @@ fn ext_octet(input: Span) -> IResult<Span, Span> {
 
 // section = { "*" ~ ASCII_DIGIT+ }
 fn section(input: Span) -> IResult<Span, u32> {
-    context(
-        "section",
-        preceded(char('*'), nom::character::complete::u32),
-    )
-    .parse(input)
+    context("section", preceded(tag("*"), nom::character::complete::u32)).parse(input)
 }
 
 // regular_parameter = { attribute ~ cfws? ~ "=" ~ cfws? ~ value }
@@ -1417,7 +1494,7 @@ fn regular_parameter(input: Span) -> IResult<Span, MimeParameter> {
     context(
         "regular_parameter",
         map(
-            (attribute, opt(cfws), char('='), opt(cfws), value),
+            (attribute, opt(cfws), tag("="), opt(cfws), value),
             |(name, _, _, _, value)| MimeParameter {
                 name: name.as_bytes().into(),
                 value: value.as_bytes().into(),
@@ -1448,47 +1525,49 @@ fn value(input: Span) -> IResult<Span, BString> {
 pub struct Parser;
 
 impl Parser {
-    pub fn parse_mailbox_list_header(text: &str) -> Result<MailboxList> {
+    pub fn parse_mailbox_list_header(text: &[u8]) -> Result<MailboxList> {
         parse_with(text, mailbox_list)
     }
 
-    pub fn parse_mailbox_header(text: &str) -> Result<Mailbox> {
+    pub fn parse_mailbox_header(text: &[u8]) -> Result<Mailbox> {
         parse_with(text, mailbox)
     }
 
-    pub fn parse_address_list_header(text: &str) -> Result<AddressList> {
+    pub fn parse_address_list_header(text: &[u8]) -> Result<AddressList> {
         parse_with(text, address_list)
     }
 
-    pub fn parse_msg_id_header(text: &str) -> Result<MessageID> {
+    pub fn parse_msg_id_header(text: &[u8]) -> Result<MessageID> {
         parse_with(text, msg_id)
     }
 
-    pub fn parse_msg_id_header_list(text: &str) -> Result<Vec<MessageID>> {
+    pub fn parse_msg_id_header_list(text: &[u8]) -> Result<Vec<MessageID>> {
         parse_with(text, msg_id_list)
     }
 
-    pub fn parse_content_id_header(text: &str) -> Result<MessageID> {
+    pub fn parse_content_id_header(text: &[u8]) -> Result<MessageID> {
         parse_with(text, content_id)
     }
 
-    pub fn parse_content_type_header(text: &str) -> Result<MimeParameters> {
+    pub fn parse_content_type_header(text: &[u8]) -> Result<MimeParameters> {
         parse_with(text, content_type)
     }
 
-    pub fn parse_content_transfer_encoding_header(text: &str) -> Result<MimeParameters> {
+    pub fn parse_content_transfer_encoding_header(text: &[u8]) -> Result<MimeParameters> {
         parse_with(text, content_transfer_encoding)
     }
 
-    pub fn parse_unstructured_header(text: &str) -> Result<BString> {
+    pub fn parse_unstructured_header(text: &[u8]) -> Result<BString> {
         parse_with(text, unstructured)
     }
 
-    pub fn parse_authentication_results_header(text: &str) -> Result<AuthenticationResults> {
+    pub fn parse_authentication_results_header(text: &[u8]) -> Result<AuthenticationResults> {
         parse_with(text, authentication_results)
     }
 
-    pub fn parse_arc_authentication_results_header(text: &str) -> Result<ARCAuthenticationResults> {
+    pub fn parse_arc_authentication_results_header(
+        text: &[u8],
+    ) -> Result<ARCAuthenticationResults> {
         parse_with(text, arc_authentication_results)
     }
 }
@@ -1549,7 +1628,7 @@ pub struct AuthenticationResults {
 
 /// Emits a value that was parsed by `value`, into target
 fn emit_value_token(value: &[u8], target: &mut Vec<u8>) {
-    let use_quoted_string = !value.iter().all(|&c| is_mime_token(c as char) || c == b'@');
+    let use_quoted_string = !value.iter().all(|&c| is_mime_token(c) || c == b'@');
     if use_quoted_string {
         target.push(b'"');
         for (start, end, c) in value.char_indices() {
@@ -1625,7 +1704,7 @@ impl AddrSpec {
     }
 
     pub fn parse(email: &str) -> Result<Self> {
-        parse_with(email, addr_spec)
+        parse_with(email.as_bytes(), addr_spec)
     }
 }
 
@@ -1633,10 +1712,7 @@ impl EncodeHeaderValue for AddrSpec {
     fn encode_value(&self) -> SharedString<'static> {
         let mut result: Vec<u8> = vec![];
 
-        let needs_quoting = !self
-            .local_part
-            .iter()
-            .all(|&c| is_atext(c as char) || c == b'.');
+        let needs_quoting = !self.local_part.iter().all(|&c| is_atext(c) || c == b'.');
         if needs_quoting {
             result.push(b'"');
             // RFC5321 4.1.2 qtextSMTP:
@@ -1714,22 +1790,28 @@ pub struct Mailbox {
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 #[serde(transparent)]
-pub struct MessageID(pub String);
+pub struct MessageID(pub BString);
 
 impl EncodeHeaderValue for MessageID {
     fn encode_value(&self) -> SharedString<'static> {
-        format!("<{}>", self.0).into()
+        let mut result = Vec::<u8>::with_capacity(self.0.len() + 2);
+        result.push(b'<');
+        result.push_str(&self.0);
+        result.push(b'>');
+        result.into()
     }
 }
 
 impl EncodeHeaderValue for Vec<MessageID> {
     fn encode_value(&self) -> SharedString<'static> {
-        let mut result = String::new();
+        let mut result = BString::default();
         for id in self {
             if !result.is_empty() {
                 result.push_str("\r\n\t");
             }
-            result.push_str(&format!("<{}>", id.0));
+            result.push(b'<');
+            result.push_str(&id.0);
+            result.push(b'>');
         }
         result.into()
     }
@@ -1960,12 +2042,12 @@ impl EncodeHeaderValue for MimeParameters {
                     result.push_str(&format!(";\r\n\t{name}=\"{encoded}\""));
                 }
                 MimeParameterEncoding::None | MimeParameterEncoding::Rfc2231 => {
-                    let needs_encoding = value.chars().any(|c| !is_mime_token(c) || !c.is_ascii());
+                    let needs_encoding = value.iter().any(|&c| !is_mime_token(c) || !c.is_ascii());
                     // Prefer to use quoted_string representation when possible, as it doesn't
                     // require any RFC 2231 encoding
                     let use_quoted_string = value
-                        .chars()
-                        .all(|c| (is_qtext(c) || is_quoted_pair(c)) && c.is_ascii());
+                        .iter()
+                        .all(|&c| (is_qtext(c) || is_quoted_pair(c)) && c.is_ascii());
 
                     let mut params = vec![];
                     let mut chars = value.char_indices().peekable();
@@ -1994,7 +2076,10 @@ impl EncodeHeaderValue for MimeParameters {
                                     encoded.push(b'\\');
                                 }
                                 encoded.push_str(s);
-                            } else if is_mime_token(c) && (!needs_encoding || c != '%') {
+                            } else if (c as u32) <= 0xff
+                                && is_mime_token(c as u32 as u8)
+                                && (!needs_encoding || c != '%')
+                            {
                                 encoded.push_str(s);
                             } else {
                                 for b in s.bytes() {
@@ -2175,12 +2260,17 @@ fn test_qp_encode() {
 /// string is enclosed in quotes.
 fn quote_string(s: impl AsRef<[u8]>) -> BString {
     let s = s.as_ref();
-    if s.chars().any(|c| !is_atext(c)) {
+
+    if s.iter().any(|&c| !is_atext(c)) {
         let mut result = Vec::<u8>::with_capacity(s.len() + 4);
         result.push(b'"');
         for (start, end, c) in s.char_indices() {
-            if !c.is_ascii_whitespace() && !is_qtext(c) && !is_atext(c) {
-                result.push(b'\\');
+            let c = c as u32;
+            if c <= 0xff {
+                let c = c as u8;
+                if !c.is_ascii_whitespace() && !is_qtext(c) && !is_atext(c) {
+                    result.push(b'\\');
+                }
             }
             result.push_str(&s[start..end]);
         }
@@ -2357,19 +2447,12 @@ Some(
 InvalidHeaderValueDuringGet {
     header_name: "Sender",
     error: HeaderParse(
-        "0: at line 1:
+        "Error at line 1, expected "@" but found ".":
 hello..there@docomo.ne.jp
      ^___________________
-expected '@', found .
-
-1: at line 1, in addr_spec:
-hello..there@docomo.ne.jp
-^________________________
-
-2: at line 1, in mailbox:
-hello..there@docomo.ne.jp
-^________________________
 
+while parsing addr_spec
+while parsing mailbox
 ",
     ),
 }
diff --git a/crates/mailparsing/src/strings.rs b/crates/mailparsing/src/strings.rs
index a8f09daa..ff965bd4 100644
--- a/crates/mailparsing/src/strings.rs
+++ b/crates/mailparsing/src/strings.rs
@@ -107,11 +107,6 @@ impl SharedString<'_> {
         String::from_utf8_lossy(self.as_bytes())
     }
 
-    #[deprecated = "use either as_bytes() or to_str() instead"]
-    pub fn as_str(&self) -> &str {
-        self.to_str().expect("SharedString is not UTF-8")
-    }
-
     pub fn len(&self) -> usize {
         match self {
             Self::Owned(s) => s.len(),