Messages from Mailer-Daemon handled differently?

[Paul]

I have a MailScanner box that scans for spam using SpamAssassin and does
some file attachment checks before passing messages to a box running
MailSweeper for virus scanning and futher processing.

Prior to changing my "X-MailScanner: Found to be clean" header I found a
couple of Sobig.F infected messages that the MailScanner box didn't reject
on the basis of their *.pif attachments (though the MailSweeper box did).

Both messages had *.pif attachments, "document_9446.pif" and
"document_all.pif" respectively.

Both were delivery failure notices generated where an infected machine had
sent the Sobig.F virus using one of our email addresses as the sender.  In
both cases the mail servers bounced the message back and the messages came
from Mailer-Daemon @ each of the recipient domains.

Does MailScanner handle mail from Mailer-Daemon at ... differently, I wonder?