mail corruption cc-ing multiple recpts

I have a customer who runs 3 Exchange 2000 servers each with McAfee groupshield and Viruscan 7. Several times a week if they send an internal email and cc it to several recipients between two of the three sites, all the cc’d mails become corrupt and can only be deleted via OWA. Also if there are attachments, these may be stripped off.
Any ideas ?!