well. this user stopped complaining a few days ago, and then we tried the Outlook Connector for him; everything seems to be working smoothly on his machine.
In an unfortuante turn of events, it appears that I am having a similar problem now. I recently switched from Blackberry to Treo, and tried using ChatterMail (as i read in another post); and shortly after doing that got the same symptoms as he was having.
Chattermail became unable to login to the server; and after getting home, my web client hangs on login like his did.
If i had to guess, it seems like this is related to persistent IMAP connections; as Chatter and (i'm guessing) Outlook IMAP keep the connection open for extended periods of time. (of course, just a guess.)
Anyway.. reading in the logs (/opt/zimbra/zimbra.log) it appears that the problem is related to :
restarting tomcat has fixed the problem for now.
2005-12-10 12:24:18,227 INFO [Convertd-369]  ProtocolHandler - Exception occurred while handling connection
at EDU.oswego.cs.dl.util.concurrent.PooledExecutor$Worker.run(Unknown Source)