We have an Exchange 2003 server that has been running on our Windows SBS 2003 server for several years without issue. All virus protection is up-to-date and a full scan found no issues. We have Avast Business Protection Plus which has the Exchange filter. A full malware scan also found nothing.
I have disabled the Outgoing Mail and and have created a temporary SMTP connector to funnel all messages to which is not scheduled to send until 12 hours from now. Inspection of the messages found that they are all being sent by one address which is: firstname.lastname@example.org. This email address is not associated in any way with out domain.
Is there a way to stop this?