Which error number do you get, 56 or 32? If you do a search on Symantec's site for scan error followed by the error number they have some info, although you may have already done this.
I see scan error 56 often but it has only been from Spam sources. After talking to Symantec a few times I am just going to go ahead and upgrade to the new version, Mail Security for SMPT 4.0. The only real workaround I got was to allow the malformed messages to be delivered or held until I check them, and neither appealed to me. Sorry can't be more help but if find an ideal solution or info that it is from the sending client please post details I would like to know. Good luck