Distribution list attachments scrambled for some users

We occasionally experience a strange problem. Sometimes when a user sends a couple of attachments to a distribution list containing offsite email addresses, some of those users will receive the attachments, and some will receive garbage.
I would suspect a problem on their end except: When the email is resent to the problemed users individually instead of through the distribution list, they receive and can open the attachments fine.

Any ideas?

Our setup: Windows Server 2000 service Pack 4. Exchange 2000 service pack 3.
GFI Mail Essentials, McAfee Group shield
Client Windows XP with Outlook 2000.