diff --git a/example/example.dart b/example/example.dart
index 0a5e9eea..010da4df 100644
--- a/example/example.dart
+++ b/example/example.dart
@@ -37,7 +37,10 @@ void main() async {
   await smtpClient.ehlo();
   if (smtpClient.serverInfo.supportsAuth(AuthMechanism.plain)) {
     await smtpClient.authenticate(
-        'user@example.com', 'password', AuthMechanism.plain);
+      'user@example.com',
+      'password',
+      AuthMechanism.plain,
+    );
   }
 
   final builder = MessageBuilder()
diff --git a/lib/src/codecs/mail_codec.dart b/lib/src/codecs/mail_codec.dart
index 9eceeb6a..539b4d1e 100644
--- a/lib/src/codecs/mail_codec.dart
+++ b/lib/src/codecs/mail_codec.dart
@@ -410,16 +410,20 @@ abstract class MailCodec {
       return text;
     }
     final buffer = StringBuffer();
-    final runes = text.runes;
+    final runes = text.runes.toList(growable: false);
     int? lastRune;
     int? lastSpaceIndex;
     var currentLineLength = 0;
     var currentLineStartIndex = 0;
     for (var runeIndex = 0; runeIndex < runes.length; runeIndex++) {
-      final rune = runes.elementAt(runeIndex);
+      final rune = runes[runeIndex];
       if (rune == AsciiRunes.runeLineFeed &&
           lastRune == AsciiRunes.runeCarriageReturn) {
-        buffer.write(text.substring(currentLineStartIndex, runeIndex + 1));
+        buffer.write(
+          String.fromCharCodes(
+            runes.sublist(currentLineStartIndex, runeIndex + 1),
+          ),
+        );
         currentLineLength = 0;
         currentLineStartIndex = runeIndex + 1;
         lastSpaceIndex = null;
@@ -433,7 +437,7 @@ abstract class MailCodec {
           // edge case: this could be in the middle of a \r\n sequence:
           if (rune == AsciiRunes.runeCarriageReturn &&
               runeIndex < runes.length - 1 &&
-              runes.elementAt(runeIndex + 1) == AsciiRunes.runeLineFeed) {
+              runes[runeIndex + 1] == AsciiRunes.runeLineFeed) {
             lastRune = rune;
             continue; // the break will be handled in the next loop iteration
           }
@@ -444,7 +448,11 @@ abstract class MailCodec {
             endIndex++;
           }
           buffer
-            ..write(text.substring(currentLineStartIndex, endIndex))
+            ..write(
+              String.fromCharCodes(
+                runes.sublist(currentLineStartIndex, endIndex),
+              ),
+            )
             ..write('\r\n');
           currentLineLength = 0;
           currentLineStartIndex = endIndex;
@@ -454,8 +462,8 @@ abstract class MailCodec {
       lastRune = rune;
     }
 
-    if (currentLineStartIndex < text.length) {
-      buffer.write(text.substring(currentLineStartIndex));
+    if (currentLineStartIndex < runes.length) {
+      buffer.write(String.fromCharCodes(runes.sublist(currentLineStartIndex)));
     }
 
     return buffer.toString();
diff --git a/lib/src/imap/imap_client.dart b/lib/src/imap/imap_client.dart
index 139f3e26..2d90f3ec 100644
--- a/lib/src/imap/imap_client.dart
+++ b/lib/src/imap/imap_client.dart
@@ -345,7 +345,7 @@ class ImapClient extends ClientBase {
     // the continuation would hang forever since onConnectionError is not
     // invoked on an expected disconnect.
     _failPendingIdleContinuation('client disconnected');
-    _eventController.close();
+    await _eventController.close();
 
     return super.disconnect();
   }
@@ -2762,7 +2762,8 @@ class ImapClient extends ClientBase {
     }
     if (_isInIdleMode) {
       // `+ idling` from the server -- IDLE mode is now truly active.
-      // Resolve any pending completer from idleStart(waitForContinuation: true).
+      // Resolve any pending completer from
+      // idleStart(waitForContinuation: true).
       final completer = _idleContinuationCompleter;
       if (completer != null && !completer.isCompleted) {
         _idleContinuationCompleter = null;
diff --git a/lib/src/message_builder.dart b/lib/src/message_builder.dart
index 906f1962..90bc1db8 100644
--- a/lib/src/message_builder.dart
+++ b/lib/src/message_builder.dart
@@ -1437,10 +1437,13 @@ class MessageBuilder extends PartBuilder {
         'Return-Receipt-To',
       );
       if (recipient == null || recipient.isEmpty) {
+        final headerValue = originalMessage.getHeaderValue(
+          MailConventions.headerDispositionNotificationTo,
+        );
         throw InvalidArgumentException(
-          'Invalid header ${MailConventions.headerDispositionNotificationTo} '
-          'in message: '
-          '${originalMessage.getHeaderValue(MailConventions.headerDispositionNotificationTo)}',
+          'Invalid header '
+          '${MailConventions.headerDispositionNotificationTo} in message: '
+          '$headerValue',
         );
       }
     }
@@ -1503,7 +1506,9 @@ class MessageBuilder extends PartBuilder {
       return '>\r\n';
     }
 
-    return '>${header.split('\r\n').join('\r\n>')}\r\n>${text.split('\r\n').join('\r\n>')}';
+    final quotedHeader = header.split('\r\n').join('\r\n>');
+    final quotedText = text.split('\r\n').join('\r\n>');
+    return '>$quotedHeader\r\n>$quotedText';
   }
 
   /// Generates a message ID
diff --git a/lib/src/mime_message.dart b/lib/src/mime_message.dart
index 36c0b6a0..22ac2d71 100644
--- a/lib/src/mime_message.dart
+++ b/lib/src/mime_message.dart
@@ -614,9 +614,14 @@ class MimeMessage extends MimePart {
     this.guid = guid;
   }
 
-  int? xGmThrid; // X-GM-THRID
-  int? xGmMsgid; // X-GM-MSGID
-  List<String>? xGmLabels; // X-GM-LABELS
+  /// The Gmail thread ID (X-GM-THRID)
+  int? xGmThrid;
+
+  /// The Gmail message ID (X-GM-MSGID)
+  int? xGmMsgid;
+
+  /// The Gmail labels (X-GM-LABELS)
+  List<String>? xGmLabels;
 
   /// The modifications sequence of this message.
   ///
diff --git a/lib/src/smtp/smtp_client.dart b/lib/src/smtp/smtp_client.dart
index 304d13f8..a28d79ae 100644
--- a/lib/src/smtp/smtp_client.dart
+++ b/lib/src/smtp/smtp_client.dart
@@ -122,7 +122,9 @@ class SmtpClient extends ClientBase {
   ///
   /// Usage:
   /// ```dart
-  /// smtpClient.eventStream.whereType<SmtpConnectionLostEvent>().listen((event) {
+  /// smtpClient.eventStream
+  ///   .whereType<SmtpConnectionLostEvent>()
+  ///   .listen((event) {
   ///   _log(event.type);
   /// });
   /// ```
diff --git a/test/message_builder_test.dart b/test/message_builder_test.dart
index 8c05fc43..04b2f56c 100644
--- a/test/message_builder_test.dart
+++ b/test/message_builder_test.dart
@@ -1748,6 +1748,35 @@ END:VCARD\r
       //expect(part?.decodeDispositionNotification())
     });
   });
+
+  group('performance', () {
+    test('buildMimeMessage should be fast with 100KB body', () {
+      final builder = MessageBuilder.prepareMultipartAlternativeMessage()
+        ..from = [const MailAddress('Sender', 'sender@example.com')]
+        ..to = [const MailAddress('Recipient', 'recipient@example.com')]
+        ..subject = 'Large body perf test';
+
+      final largeBody = 'A' * (100 * 1024);
+
+      builder.addTextPlain(largeBody);
+
+      final sw = Stopwatch()..start();
+      final mime = builder.buildMimeMessage();
+      sw.stop();
+
+      // Sanity checks
+      expect(mime, isNotNull);
+      expect(mime.renderMessage().length, greaterThan(100000));
+
+      expect(
+        sw.elapsedMilliseconds,
+        lessThan(1000),
+        reason:
+            'buildMimeMessage() should complete in <1s, but currently '
+            'takes ${sw.elapsedMilliseconds}ms for a 100KB body. ',
+      );
+    });
+  });
 }
 
 const String complexMessageText = '''