I too have the problem with MessageLabs (Symantec) blocking my email to a customer because of the attachments – “unacceptable attachments” according to Symantec. The attachments are simple PDF files which have been transmitted without a problem each month for several months up to now.
No satisfaction from Symantec of course and my customer IT dept is not interested.

Really interesting is that the blocking only occurs when I send from my MAC, not when I send from my PC. Yet the attachments are the same.

If we all complain to our customers about Symantec maybe they will see a problem and look at using a different company.