It seems more logical to use the SERVER_NAME as the RHS of the Message-ID

than to use the remote host from which the user is connecting. Even more
since the REMOTE_ADDR is already contained within the Message-ID.

class/deliver/Deliver.class.php

@@ -379,7 +379,7 @@ class Deliver {
         $date = date('D, j M Y H:i:s ', mktime()) . $this->timezone();
         /* Create a message-id */
         $message_id = '<' . $REMOTE_PORT . '.' . $REMOTE_ADDR . '.';
-        $message_id .= time() . '.squirrel@' . (isset($REMOTE_HOST) ? $REMOTE_HOST : "[$REMOTE_ADDR]") .'>';
+        $message_id .= time() . '.squirrel@' . $SERVER_NAME .'>';
         /* Make an RFC822 Received: line */
         if (isset($REMOTE_HOST)) {
             $received_from = "$REMOTE_HOST ([$REMOTE_ADDR])";