I've researched this quite a bit on the site and found that the answers don't work for me, however my problem seems to be differenet than the others.
I'm running Outlook 2007 on Windows XP, SP 3. Everything is 100% fully up-to-date.
Randomly (about 1/4 of the time) when one of users saves an email straight to a networked hard drive, she'll receive the following message:
"The messaging inteface has returned an unknown error. If this problem persists, restart Outlook".
Naturally, have restarted Outlook, and her computer. Still have the same problem.