You may also want to confirm SMTP Authentication is enabled in the domain settings, within Settings>Protocols>SMTP In, and within your domain defaults. If you are not requiring authentication on SMTP this might be where the spam is originating from. If you need further assistance tracking this down please don't hesitate to submit a ticket with support. Have a good one!
Kyle Kerst
IT Coordinator
SmarterTools Inc.
www.smartertools.com