Over this last weekend we had a one user account that started receiving mass amounts of returned spam emails with MAILER-DAEMON (Mailer Delivery System) and System Undeliverable. I have been battling spam for a while and have managed to keep it pretty minimal now with IMF as well as a Sonicwall firewall appliance with Spam filtering. I have read a few questions related to this one and have tried many of the suggestions that anyone may have had such as creating an SPF record and assuring my dns and reverse dns are correct. I do not have open relay and I am using recipeint filtering have enabled tarpit to my registry. I have scanned the particular users workstations with Kaspersky Anti Virus, Malwarebytes, and a few other antispyware apps. Last thing I have done was changed all the administrator passwords as well as most of our users passwords on our domain.
I am looking for any open suggestions on this issue or if it's something simple that I may have overlooked.