Intermittent password prompt from IMAP access on local network

We have a couple users who use IMAP through Apple Mail (using OS X Leopard) to connect to our Exchange server both inside and outside the corporate network. While connected on the local network, they get prompted to enter their password several times a day, even though it is saved in the system, and the app is checking mail every few minutes. Re-entering the password does not have any affect on this. I setup an IMAP account on a PC through Outlook, and it gets the intermittent password prompts as well. I tried setting the server time-out on the account in Outlook to a couple minutes longer, and it is still prompting from time to time. Email is coming through the server just fine, and the clients are receiving the mail from the server.

I am assuming this is some sort of time-out when the client is requesting to connect to the server, but I unsure of the cause. I rebooted the servers and it seemed to give a temporary effect in not being prompted for the passwords, but within a few hours the password prompts come back. This problem just came up last week, and there have not been any configuration changes to the software or hardware recently. I can access and ping all of the servers on the internal network with virtually no delay, so it does not seem to be network latency. Is there something in Exchange itself that might be causing a time-out like this? A setting i can tweak so it will try longer before kicking back the password prompt?Start Free Trial