docs: clarify/add the recommendation to set AutoSubmitted if needed

FAQ.md

@@ -1124,6 +1124,8 @@ the same way you'd create any other message. There are only a few slight differe
 3. You will want to copy the original message's `References` header into the reply message's
    `References` header and then append the original message's `Message-Id` header.
 4. You will probably want to "quote" the original message's text in the reply.
+5. If you are generating an automatic reply, you should also follow [RFC3834](https://www.rfc-editor.org/rfc/rfc3834)
+   and set the `Auto-Submitted` value to `auto-replied`.
 
 If this logic were to be expressed in code, it might look something like this:
 
@@ -1163,6 +1165,9 @@ public static MimeMessage Reply (MimeMessage message, MailboxAddress from, bool
         reply.References.Add (message.MessageId);
     }
 
+    // if this is an automatic reply, be sure to specify this using the Auto-Submitted header in order to avoid (infinite) mail loops
+    reply.Headers.Add (HeaderId.AutoSubmitted, "auto-replied");
+
     // quote the original message text
     using (var quoted = new StringWriter ()) {
         var sender = message.Sender ?? message.From.Mailboxes.FirstOrDefault ();