diff --git a/lib/mail_conventions.dart b/lib/mail_conventions.dart
index 04541c09..db18afe6 100644
--- a/lib/mail_conventions.dart
+++ b/lib/mail_conventions.dart
@@ -71,6 +71,7 @@ class MailConventions {
       'Content-Transfer-Encoding';
   static const String headerContentDisposition = 'Content-Disposition';
   static const String headerContentDescription = 'Content-Description';
+  static const String headerMimeVersion = 'MIME-Version';
 //static const String header = '';
 
 }
diff --git a/lib/message_builder.dart b/lib/message_builder.dart
index 7694e63f..52f148b6 100644
--- a/lib/message_builder.dart
+++ b/lib/message_builder.dart
@@ -329,6 +329,7 @@ class MessageBuilder extends PartBuilder {
     if (subject != null) {
       addHeader('Subject', subject, encode: true);
     }
+    addHeader(MailConventions.headerMimeVersion, '1.0');
     if (replyToMessage != null) {
       var originalMessageId = replyToMessage.getHeaderValue('message-id');
       addHeader(MailConventions.headerInReplyTo, originalMessageId);