Zend_Mail and Microsoft Exchange Servers

Has anyone experienced problems with sending attachment with Zend_Mail
to MS Exchange servers? I consistently get emails from them saying the
attachment on the email I sent was corrupt.

I initially thought the PDF was corrupt, so I started saving a copy of
it before emailing it. The PDF opens fine, and weirder yet, sending
the attachment with my email client (Evolution) works fine, no
corruption issues.

I'd test this myself, but I don't have an MS Exchange server to run
the tests on. I might be able to send a couple more emails to one of
the users, but they're typically too busy to expect any kind of
reasonable turn around time.