I have a working IMAP setup which has lots of users behind it. IMAP is working well for all of them except those users using MS Outlook Express 6 (the version bundled with Windows XP) or MS Outlook 6 (the version bundled with Office XP). For those users, Outlook often just returns "unknown error communicating with server." This occurs with both IMAP and IMAPS.
Sounds like identd timeout. Can you edit the xinetd config file for one of the imap services and remove the USERID directive and then restart xinetd. See if that solves the problem. It has solved the problem for other users, so it is likely the problem you are experiencing.
I don't think that's the problem. I've changed /etc/xinetd.d/imap{,s} and I still get the error. Furthermore, the windoze boxes are running identd, so it's not likely to be the problem anyway....
I don't have access to Windows XP, nor Outlook Express 6, and am unable to test it out. I am more than willing to expect that this is a bug in Outlook itself than a bug in the imap server. Outlook is probably one of the most widely used email clients, and if nobody using Outlook could access UW imap server on a Red Hat Linux 7.2 box, I'd have received a large number of bug reports by now. Since I haven't, I'm going to venture that it is a localized issue or an Outlook issue. If the problem is still present, I recommend upgrading to our latest imap package erratum if you haven't already, and also upgrading to the latest version of Microsoft Outlook Express or possibly downgrading. If all else fails, then I suggest reporting the problem to the UW imap team directly.