Emails as Attachments

Not sure if this is right group for this or not.
I have problem that my system's emails are sometimes sent out
attachments (.eml format).
Not sure why or where it is happening.

I have the following configuration:
Outlook 2000 clients with Norton AnitViurs or Symantec Client Security
- MS Exchange 5.5 server
- Trendmicro Interscan VirusWall smtp gateway
- ISP SMTP.

I had a Debian Linux box running Postfix, Clamav, Amavis as another
gateway but I took that out of the loop. It made no difference.

Not sure but this is driving me nuts since it affects all types of
emails (Text, HTML, Rich Text) but not always.

J