Nope - but thanks for the thought. I use AVG myself... will watch for that one!

This was happening on every email with a JPG attachment until we disabled the outgoing AV check. Unless she deleted the offending emails, they would continue to be sent over and over again (with the original stuck in the outbox) even after a cold reboot. The messages didn't appear in 'Sent Items', either. I received a test email 15 times this morning before she deleted the original!

Emails without JPGs were behaving normally, which was why I suspected the AV software had something to do with it.