Hey Chuck,
This has actually been brought up in a couple other threads so I understand the concern. We have a couple ways that we can attempt to do this, however, what is going to help the most is going to be version 8 of the SBG (Symantec Brightmail Gateway). We will be implementing BATV which helps eliminate backscatter spam.
So until this comes out there are some things you can do. Instead of restating I'll post a link to another thread. It's kinda hard to explain. :smileyhappy:
https://forums.symantec.com/syment/board/message?board.id=AVGateway&thread.id=1191
Let me know if you need any clarification or if you have any questions. I hope this helps!
Thanks!
Tom