I think what is happening here from viewing on the 2003 servers I've been installing is an e-mail is coming in to an unknown address for example
[email protected], as the e-mail server cannot deliver it attempts to send back to the sender (eg
[email protected]).
This e-mail will be coming from
[email protected], if you are using DNS to send e-mails from your server and the return domain has been removed/doesn't exist, the mail server will create a queue and attempt to re-send.
So, its not a relay attempt, merely an NDR to a non-existent domain.