diff --git a/src/message/header/textual.rs b/src/message/header/textual.rs
index 7c5a0e6..f880130 100644
--- a/src/message/header/textual.rs
+++ b/src/message/header/textual.rs
@@ -65,7 +65,7 @@ text_header!(
 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")
+    Header(MessageId, "Message-ID")
 );
 text_header!(
     /// `User-Agent` header. Contains information about the client,
diff --git a/src/message/mod.rs b/src/message/mod.rs
index b09e614..34adce1 100644
--- a/src/message/mod.rs
+++ b/src/message/mod.rs
@@ -375,7 +375,7 @@ impl MessageBuilder {
         self.header(header::References::from(id))
     }
 
-    /// Set [Message-Id
+    /// Set [Message-ID
     /// header](https://tools.ietf.org/html/rfc5322#section-3.6.4)
     ///
     /// Should generally be inserted by the mail relay.