First, what components of SEP are installed and in use?
The first one that comes to mind is the firewall component but I won't speculate. Any traffic blocks would show up in the Traffic log though. This article can be referred for testing application issues with the SEP firewall:
http://www.symantec.com/docs/TECH203497
I would start by installing SEP with only the AV component to see what the result is. You can add back each additional component one at a time to see which one is the culprit.
Based on this article:
http://www.symantec.com/docs/TECH102400
...SEP should automatically detect and add AV exclusions for Exchange.