We have a fax server setup via a usb modem on one of our server 2003 machines.
Rather than printing it off we have it setup to route through email.
For some unknown reason most of them go through to the email, but some get stuck in the inbox.
There is now pattern and so far we can't see any reason for it.
It's only a small percentage like 5% that get stuck, if that, but its still annoying cos we have to get people to re send them.