Assuming the problem is not the network, you might want to look into
http://support.microsoft.com/kb/329067/en-us
or
http://support.microsoft.com/kb/947699
I'm trying to diagnose a similar problem: We have two new iMacs (and one older Mac) on the office LAN that recently have started disconnecting from the Exchange Server. The machines, without any warning message, simply disconnect, and next to the user's account in Entourage it says "(Not connected)". When the user tries to send email, it just sits in the outbox. If they quit Entourage and restart, they report that it doesn't connect at all, then. The only thing that seems to work is rebooting the machine.
I'm going in Friday to take a look at the machines, but was wondering if anyone has any suggestions, to make my site visit most productive.
In my case, after reviewing
http://support.microsoft.com/kb/947699 I'm skeptical that this is our problem because (a) this suggests that sending email will prompt it to reconnect, which it doesn't in this case; and (b) IT reports no server outages.
I think
http://support.microsoft.com/kb/329067/en-us is more promising (though I'm waiting for IT to confirm).
FYI, users are running Mac OS X 10.6.2 with Entourage 2008 12.2.0. Server is running Exchange Server 2003 with latest service packs. Not running EWS (Exchange Web Services) because we're not running Exchange 2007 (yet). I'll do the updates to get the client machines up to date, but I don't see anything in the release notes for the subsequent updates that say anything about this issue.
I would definitely appreciate any suggestions that anyone might have.