Here's an odd one.
Intermittently our Exchange 2000 (Standard) box shoots emails into one of the senior management's Blackberry handheld device and they (sometimes) vanish from their email client inbox.
I've witnessed this myself and the really odd part is when the message pops up briefly in Outlook as a new email then vanishes a split second later. It appears on the Blackberry around the same time.
The messages in question are from different users/networks/servers and no single sender seems to trigger this behaviour.
We seem to have messages going AWOL at present where several senders have asked why they have had no reply to their emails which were never received at our Exchange server.
Spam control is through a Message labs partner.
One final point which may (or may not) be of relevance: we have started seeing regular (attempting to connect to the exchange server) messages when Outlook is left open for long periods.
The client connection to the server seems to drop, but ping requests to the Exchange box constantly reply through these dropped connections.
I've no clue whether they are all related or not, but I thought I'd throw lots of detail into the mix in the hopes it may help you wonderful people build a picture of what may be going on.