Is it possible that what you are seeing is just stacked up in the mail queue? I had a similar problem with Postfix. I was an open relay for one day and spent 4 days tracking down where all these messages in my logs were coming from even after I was certain I was no longer and open relay. turned out the mail queue was trying to send the backed up spam to addresses that were no longer valid or rejecting the spam. Once I cleaned out my mail queue everything was back to normal.

Not sure if this will help, but just a thought.

BTW I'm not sure how to reply and get this quoted Outlook kept wanting to reply to people's addresses.