diff --git a/src/address/envelope.rs b/src/address/envelope.rs
index e458c58..6d5c299 100644
--- a/src/address/envelope.rs
+++ b/src/address/envelope.rs
@@ -112,7 +112,7 @@ impl TryFrom<&Headers> for Envelope {
     fn try_from(headers: &Headers) -> Result<Self, Self::Error> {
         let from = match headers.get::<header::Sender>() {
             // If there is a Sender, use it
-            Some(header::Sender(a)) => Some(a.email.clone()),
+            Some(sender) => Some(Mailbox::from(sender.clone()).email),
             // ... else try From
             None => match headers.get::<header::From>() {
                 Some(header::From(a)) => {
diff --git a/src/lib.rs b/src/lib.rs
index a8067af..5704ca3 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -122,10 +122,6 @@ mod executor;
 pub mod message;
 pub mod transport;
 
-#[cfg(feature = "builder")]
-#[macro_use]
-extern crate hyperx;
-
 #[cfg(feature = "async-std1")]
 pub use self::executor::AsyncStd1Executor;
 #[cfg(all(any(feature = "tokio02", feature = "tokio1", feature = "async-std1")))]
@@ -208,9 +204,9 @@ mod test {
         let to = Mailboxes::new().with("amousset@example.com".parse().unwrap());
 
         let mut headers = Headers::new();
-        headers.set(header::From(from));
-        headers.set(header::Sender(sender));
-        headers.set(header::To(to));
+        headers.set(header::From::from(from));
+        headers.set(header::Sender::from(sender));
+        headers.set(header::To::from(to));
 
         assert_eq!(
             Envelope::try_from(&headers).unwrap(),
@@ -228,8 +224,8 @@ mod test {
         let sender = Mailbox::new(None, "kayo2@example.com".parse().unwrap());
 
         let mut headers = Headers::new();
-        headers.set(header::From(from));
-        headers.set(header::Sender(sender));
+        headers.set(header::From::from(from));
+        headers.set(header::Sender::from(sender));
 
         assert!(Envelope::try_from(&headers).is_err(),);
     }
diff --git a/src/message/header/content.rs b/src/message/header/content.rs
index 774689c..0f23b02 100644
--- a/src/message/header/content.rs
+++ b/src/message/header/content.rs
@@ -7,11 +7,6 @@ use std::{
     str::{from_utf8, FromStr},
 };
 
-header! {
-    /// `Content-Id` header, defined in [RFC2045](https://tools.ietf.org/html/rfc2045#section-7)
-    (ContentId, "Content-ID") => [String]
-}
-
 /// `Content-Transfer-Encoding` of the body
 ///
 /// The `Message` builder takes care of choosing the most
diff --git a/src/message/header/mailbox.rs b/src/message/header/mailbox.rs
index f5fb8c5..19a336f 100644
--- a/src/message/header/mailbox.rs
+++ b/src/message/header/mailbox.rs
@@ -17,7 +17,7 @@ macro_rules! mailbox_header {
     ($(#[$doc:meta])*($type_name: ident, $header_name: expr)) => {
         $(#[$doc])*
         #[derive(Debug, Clone, PartialEq)]
-        pub struct $type_name(pub Mailbox);
+        pub struct $type_name(Mailbox);
 
         impl Header for $type_name {
             fn header_name() -> &'static str {
@@ -39,6 +39,20 @@ macro_rules! mailbox_header {
                 f.fmt_line(&self.0.recode_name(utf8_b::encode))
             }
         }
+
+        impl std::convert::From<Mailbox> for $type_name {
+            #[inline]
+            fn from(mailbox: Mailbox) -> Self {
+                Self(mailbox)
+            }
+        }
+
+        impl std::convert::From<$type_name> for Mailbox {
+            #[inline]
+            fn from(this: $type_name) -> Mailbox {
+                this.0
+            }
+        }
     };
 }
 
@@ -46,7 +60,7 @@ macro_rules! mailboxes_header {
     ($(#[$doc:meta])*($type_name: ident, $header_name: expr)) => {
         $(#[$doc])*
         #[derive(Debug, Clone, PartialEq)]
-        pub struct $type_name(pub Mailboxes);
+        pub struct $type_name(pub(crate) Mailboxes);
 
         impl MailboxesHeader for $type_name {
             fn join_mailboxes(&mut self, other: Self) {
@@ -74,6 +88,20 @@ macro_rules! mailboxes_header {
                 format_mailboxes(self.0.iter(), f)
             }
         }
+
+        impl std::convert::From<Mailboxes> for $type_name {
+            #[inline]
+            fn from(mailboxes: Mailboxes) -> Self {
+                Self(mailboxes)
+            }
+        }
+
+        impl std::convert::From<$type_name> for Mailboxes {
+            #[inline]
+            fn from(this: $type_name) -> Mailboxes {
+                this.0
+            }
+        }
     };
 }
 
diff --git a/src/message/header/special.rs b/src/message/header/special.rs
index 4614440..4cbba74 100644
--- a/src/message/header/special.rs
+++ b/src/message/header/special.rs
@@ -4,19 +4,29 @@ use hyperx::{
 };
 use std::{fmt::Result as FmtResult, str::from_utf8};
 
-#[derive(Debug, Clone, Copy, PartialEq)]
+#[derive(Debug, Copy, Clone, PartialEq)]
 /// Message format version, defined in [RFC2045](https://tools.ietf.org/html/rfc2045#section-4)
 pub struct MimeVersion {
-    pub major: u8,
-    pub minor: u8,
+    major: u8,
+    minor: u8,
 }
 
-pub const MIME_VERSION_1_0: MimeVersion = MimeVersion { major: 1, minor: 0 };
+pub const MIME_VERSION_1_0: MimeVersion = MimeVersion::new(1, 0);
 
 impl MimeVersion {
-    pub fn new(major: u8, minor: u8) -> Self {
+    pub const fn new(major: u8, minor: u8) -> Self {
         MimeVersion { major, minor }
     }
+
+    #[inline]
+    pub const fn major(self) -> u8 {
+        self.major
+    }
+
+    #[inline]
+    pub const fn minor(self) -> u8 {
+        self.minor
+    }
 }
 
 impl Default for MimeVersion {
diff --git a/src/message/header/textual.rs b/src/message/header/textual.rs
index 586d0c5..1bd0e22 100644
--- a/src/message/header/textual.rs
+++ b/src/message/header/textual.rs
@@ -9,7 +9,7 @@ macro_rules! text_header {
     ($(#[$attr:meta])* Header($type_name: ident, $header_name: expr )) => {
         #[derive(Debug, Clone, PartialEq)]
         $(#[$attr])*
-        pub struct $type_name(pub String);
+        pub struct $type_name(String);
 
         impl Header for $type_name {
             fn header_name() -> &'static str {
@@ -31,6 +31,20 @@ macro_rules! text_header {
                 fmt_text(&self.0, f)
             }
         }
+
+        impl From<String> for $type_name {
+            #[inline]
+            fn from(text: String) -> Self {
+                Self(text)
+            }
+        }
+
+        impl AsRef<str> for $type_name {
+            #[inline]
+            fn as_ref(&self) -> &str {
+                &self.0
+            }
+        }
     };
 }
 
@@ -69,6 +83,11 @@ text_header!(
     /// defined in [draft-melnikov-email-user-agent-00](https://tools.ietf.org/html/draft-melnikov-email-user-agent-00#section-3)
     Header(UserAgent, "User-Agent")
 );
+text_header! {
+    /// `Content-Id` header,
+    /// defined in [RFC2045](https://tools.ietf.org/html/rfc2045#section-7)
+    Header(ContentId, "Content-ID")
+}
 
 fn parse_text(raw: &[u8]) -> HyperResult<String> {
     if let Ok(src) = from_utf8(raw) {
diff --git a/src/message/mod.rs b/src/message/mod.rs
index 4ade049..670d8e6 100644
--- a/src/message/mod.rs
+++ b/src/message/mod.rs
@@ -167,7 +167,7 @@
 //!                                         disposition: header::DispositionType::Inline,
 //!                                         parameters: vec![],
 //!                                     })
-//!                                     .header(header::ContentId("<123>".into()))
+//!                                     .header(header::ContentId::from(String::from("<123>")))
 //!                                     .body(image_body),
 //!                             ),
 //!                     ),
@@ -318,7 +318,8 @@ impl MessageBuilder {
     ///
     /// Shortcut for `self.header(header::Subject(subject.into()))`.
     pub fn subject<S: Into<String>>(self, subject: S) -> Self {
-        self.header(header::Subject(subject.into()))
+        let s: String = subject.into();
+        self.header(header::Subject::from(s))
     }
 
     /// Set `Mime-Version` header to 1.0
@@ -336,7 +337,7 @@ impl MessageBuilder {
     ///
     /// Shortcut for `self.header(header::Sender(mbox))`.
     pub fn sender(self, mbox: Mailbox) -> Self {
-        self.header(header::Sender(mbox))
+        self.header(header::Sender::from(mbox))
     }
 
     /// Set or add mailbox to `From` header
@@ -345,7 +346,7 @@ impl MessageBuilder {
     ///
     /// Shortcut for `self.mailbox(header::From(mbox))`.
     pub fn from(self, mbox: Mailbox) -> Self {
-        self.mailbox(header::From(mbox.into()))
+        self.mailbox(header::From::from(Mailboxes::from(mbox)))
     }
 
     /// Set or add mailbox to `ReplyTo` header
@@ -381,13 +382,13 @@ impl MessageBuilder {
     /// Set or add message id to [`In-Reply-To`
     /// header](https://tools.ietf.org/html/rfc5322#section-3.6.4)
     pub fn in_reply_to(self, id: String) -> Self {
-        self.header(header::InReplyTo(id))
+        self.header(header::InReplyTo::from(id))
     }
 
     /// Set or add message id to [`References`
     /// header](https://tools.ietf.org/html/rfc5322#section-3.6.4)
     pub fn references(self, id: String) -> Self {
-        self.header(header::References(id))
+        self.header(header::References::from(id))
     }
 
     /// Set [Message-Id
@@ -399,7 +400,7 @@ impl MessageBuilder {
     /// `<UUID@HOSTNAME>`.
     pub fn message_id(self, id: Option<String>) -> Self {
         match id {
-            Some(i) => self.header(header::MessageId(i)),
+            Some(i) => self.header(header::MessageId::from(i)),
             None => {
                 #[cfg(feature = "hostname")]
                 let hostname = hostname::get()
@@ -409,7 +410,7 @@ impl MessageBuilder {
                 #[cfg(not(feature = "hostname"))]
                 let hostname = DEFAULT_MESSAGE_ID_DOMAIN.to_string();
 
-                self.header(header::MessageId(
+                self.header(header::MessageId::from(
                     // https://tools.ietf.org/html/rfc5322#section-3.6.4
                     format!("<{}@{}>", Uuid::new_v4(), hostname),
                 ))
@@ -420,7 +421,7 @@ impl MessageBuilder {
     /// Set [User-Agent
     /// header](https://tools.ietf.org/html/draft-melnikov-email-user-agent-004)
     pub fn user_agent(self, id: String) -> Self {
-        self.header(header::UserAgent(id))
+        self.header(header::UserAgent::from(id))
     }
 
     /// Force specific envelope (by default it is derived from headers)
@@ -595,7 +596,7 @@ mod test {
             .header(header::To(
                 vec!["Pony O.P. <pony@domain.tld>".parse().unwrap()].into(),
             ))
-            .header(header::Subject("яңа ел белән!".into()))
+            .header(header::Subject::from(String::from("яңа ел белән!")))
             .body(String::from("Happy new year!"))
             .unwrap();
 
@@ -641,7 +642,7 @@ mod test {
                                 disposition: header::DispositionType::Inline,
                                 parameters: vec![],
                             })
-                            .header(header::ContentId("<123>".into()))
+                            .header(header::ContentId::from(String::from("<123>")))
                             .body(img),
                     ),
             )