HTML formatting and attachments removed on POP3 internal email

Upgraded from:

• Mail Security 2.7 to 4.2 on Exchange
• NOD32 Business Ed 3 to 4.2 on desktops

Problem is:
HTML formatted emails sent internally via POP3 on our Exchange server (SBS2003) are reformated to plain text and any attachments are removed.


Other testing:

1. HTML formatted emails sent externall via POP3 are received OK without loss of formatting or attachments.
2. All emails sent via MAPI are received fine both internally and externally. No loss of formatting or attachments.

The loss of formatting and attachments to internal email has only started since the installation of Mail Security 4 / Business Edition 4.
What is causing this?