Why can't Macintosh Exchange clients send attachments?

I have a large contingent of Macintosh users, some using Mail, some using Entourage, all with the same problem that has come up suddenly.  They can't send any email with an attachment through our exchange server.  I've done everything to diagnose or solve this problem EXCEPT reboot the Exchange server, which I am saving for an absolute last resort.  I've been searching for an answer to this, and have yet to find one.

An addendum to this; these same affected users cannot use the web mail service to attach a file to an email.
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Have you looked in their Exchange/AD profiles for permissions in case those have somehow gotten changed?
heisensteinAuthor Commented:
Actually, despite all the evidence, the problem turned out to be our Firewall.  Once we restarted it, all was well.  (Our Exchange server is outsourced)

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