diff --git a/src/address/types.rs b/src/address/types.rs
index fb3b046..481ea2d 100644
--- a/src/address/types.rs
+++ b/src/address/types.rs
@@ -211,6 +211,7 @@ impl AsRef<OsStr> for Address {
 }
 
 #[derive(Debug, PartialEq, Clone, Copy)]
+/// Errors in email addresses parsing
 pub enum AddressError {
     MissingParts,
     Unbalanced,
diff --git a/src/message/header/content.rs b/src/message/header/content.rs
index 1ca4cfb..774689c 100644
--- a/src/message/header/content.rs
+++ b/src/message/header/content.rs
@@ -7,7 +7,10 @@ use std::{
     str::{from_utf8, FromStr},
 };
 
-header! { (ContentId, "Content-ID") => [String] }
+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
 ///
diff --git a/src/message/header/mod.rs b/src/message/header/mod.rs
index c0b2deb..e7f4ca2 100644
--- a/src/message/header/mod.rs
+++ b/src/message/header/mod.rs
@@ -1,8 +1,4 @@
-/*!
-
-## Headers widely used in email messages
-
-*/
+//! Headers widely used in email messages
 
 mod content;
 mod mailbox;
diff --git a/src/message/header/special.rs b/src/message/header/special.rs
index fbb8d5d..4614440 100644
--- a/src/message/header/special.rs
+++ b/src/message/header/special.rs
@@ -5,6 +5,7 @@ use hyperx::{
 use std::{fmt::Result as FmtResult, str::from_utf8};
 
 #[derive(Debug, Clone, Copy, PartialEq)]
+/// Message format version, defined in [RFC2045](https://tools.ietf.org/html/rfc2045#section-4)
 pub struct MimeVersion {
     pub major: u8,
     pub minor: u8,
diff --git a/src/message/header/textual.rs b/src/message/header/textual.rs
index ba14141..586d0c5 100644
--- a/src/message/header/textual.rs
+++ b/src/message/header/textual.rs
@@ -6,8 +6,9 @@ use hyperx::{
 use std::{fmt::Result as FmtResult, str::from_utf8};
 
 macro_rules! text_header {
-    ( $type_name: ident, $header_name: expr ) => {
+    ($(#[$attr:meta])* Header($type_name: ident, $header_name: expr )) => {
         #[derive(Debug, Clone, PartialEq)]
+        $(#[$attr])*
         pub struct $type_name(pub String);
 
         impl Header for $type_name {
@@ -33,13 +34,41 @@ macro_rules! text_header {
     };
 }
 
-text_header!(Subject, "Subject");
-text_header!(Comments, "Comments");
-text_header!(Keywords, "Keywords");
-text_header!(InReplyTo, "In-Reply-To");
-text_header!(References, "References");
-text_header!(MessageId, "Message-Id");
-text_header!(UserAgent, "User-Agent");
+text_header!(
+    /// `Subject` of the message, defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.5)
+    Header(Subject, "Subject")
+);
+text_header!(
+    /// `Comments` of the message, defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.5)
+    Header(Comments, "Comments")
+);
+text_header!(
+    /// `Keywords` header. Should contain a comma-separated list of one or more
+    /// words or quoted-strings, defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.5)
+    Header(Keywords, "Keywords")
+);
+text_header!(
+    /// `In-Reply-To` header. Contains one or more
+    /// unique message identifiers,
+    /// defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.4)
+    Header(InReplyTo, "In-Reply-To")
+);
+text_header!(
+    /// `References` header. Contains one or more
+    /// unique message identifiers,
+    /// defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.4)
+    Header(References, "References")
+);
+text_header!(
+    /// `Message-Id` header. Contains a unique message identifier,
+    /// defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.4)
+    Header(MessageId, "Message-Id")
+);
+text_header!(
+    /// `User-Agent` header. Contains information about the client,
+    /// 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")
+);
 
 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 0ca9e5d..cfb0953 100644
--- a/src/message/mod.rs
+++ b/src/message/mod.rs
@@ -332,7 +332,7 @@ impl MessageBuilder {
 
     /// Set `Sender` header. Should be used when providing several `From` mailboxes.
     ///
-    /// https://tools.ietf.org/html/rfc5322#section-3.6.2
+    /// Defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.2).
     ///
     /// Shortcut for `self.header(header::Sender(mbox))`.
     pub fn sender(self, mbox: Mailbox) -> Self {
@@ -341,7 +341,7 @@ impl MessageBuilder {
 
     /// Set or add mailbox to `From` header
     ///
-    /// https://tools.ietf.org/html/rfc5322#section-3.6.2
+    /// Defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.2).
     ///
     /// Shortcut for `self.mailbox(header::From(mbox))`.
     pub fn from(self, mbox: Mailbox) -> Self {
@@ -350,7 +350,7 @@ impl MessageBuilder {
 
     /// Set or add mailbox to `ReplyTo` header
     ///
-    /// https://tools.ietf.org/html/rfc5322#section-3.6.2
+    /// Defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.2).
     ///
     /// Shortcut for `self.mailbox(header::ReplyTo(mbox))`.
     pub fn reply_to(self, mbox: Mailbox) -> Self {
diff --git a/src/transport/smtp/authentication.rs b/src/transport/smtp/authentication.rs
index d02f50c..37f8f7c 100644
--- a/src/transport/smtp/authentication.rs
+++ b/src/transport/smtp/authentication.rs
@@ -39,15 +39,16 @@ where
 #[derive(PartialEq, Eq, Copy, Clone, Hash, Debug)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub enum Mechanism {
-    /// PLAIN authentication mechanism
-    /// RFC 4616: https://tools.ietf.org/html/rfc4616
+    /// PLAIN authentication mechanism, defined in
+    /// [RFC 4616](https://tools.ietf.org/html/rfc4616)
     Plain,
     /// LOGIN authentication mechanism
     /// Obsolete but needed for some providers (like office365)
-    /// https://www.ietf.org/archive/id/draft-murchison-sasl-login-00.txt
+    ///
+    /// Defined in [draft-murchison-sasl-login-00](https://www.ietf.org/archive/id/draft-murchison-sasl-login-00.txt).
     Login,
-    /// Non-standard XOAUTH2 mechanism
-    /// https://developers.google.com/gmail/imap/xoauth2-protocol
+    /// Non-standard XOAUTH2 mechanism, defined in
+    /// [xoauth2-protocol](https://developers.google.com/gmail/imap/xoauth2-protocol)
     Xoauth2,
 }
 
diff --git a/src/transport/smtp/extension.rs b/src/transport/smtp/extension.rs
index fdd52a2..66372cc 100644
--- a/src/transport/smtp/extension.rs
+++ b/src/transport/smtp/extension.rs
@@ -71,15 +71,15 @@ impl ClientId {
 pub enum Extension {
     /// 8BITMIME keyword
     ///
-    /// RFC 6152: https://tools.ietf.org/html/rfc6152
+    /// Defined in [RFC 6152](https://tools.ietf.org/html/rfc6152)
     EightBitMime,
     /// SMTPUTF8 keyword
     ///
-    /// RFC 6531: https://tools.ietf.org/html/rfc6531
+    /// Defined in [RFC 6531](https://tools.ietf.org/html/rfc6531)
     SmtpUtfEight,
     /// STARTTLS keyword
     ///
-    /// RFC 2487: https://tools.ietf.org/html/rfc2487
+    /// Defined in [RFC 2487](https://tools.ietf.org/html/rfc2487)
     StartTls,
     /// AUTH mechanism
     Authentication(Mechanism),
diff --git a/src/transport/smtp/mod.rs b/src/transport/smtp/mod.rs
index aa4dff8..63f75d0 100644
--- a/src/transport/smtp/mod.rs
+++ b/src/transport/smtp/mod.rs
@@ -168,7 +168,7 @@ pub const SMTP_PORT: u16 = 25;
 pub const SUBMISSION_PORT: u16 = 587;
 /// Default submission over TLS port
 ///
-/// https://tools.ietf.org/html/rfc8314
+/// Defined in [RFC8314](https://tools.ietf.org/html/rfc8314)
 pub const SUBMISSIONS_PORT: u16 = 465;
 
 /// Default timeout