I believe I had a related problem. I was trying to set up a rule to move copies from the local sent items folder to the sent items folder on my IMAP server.

It wouldn't work and after some digging it turned out there was a bug in Outlook 2000 that broke rules in the sent items folder. I had to upgrade to the latest service release for Office and Outlook which fixed the problem.

I still have an annoying problem though, if for some reason my IMAP server is unavailable when the rule runs the sent item rule fails, but instead of just leaving the message in the local sent items folder, Outlook just bins the message (and no it doesn't put it into the delete items folder, it just throws it away).

Thankfully I don't have to use Outlook much, 99% of my mail is handled via Squirrel Mail.