We normally don't have a need for IMAP connections from outside the company network but in order to allow a one user to use IMAP on a portable device I've turned it on and opened port 993 on the firewall. When the user in question was unable to get connected I tested this using Outlook remotely.
Start by creating a new IMAP account in Outlook using a test account. No problems, it worked perfectly. Now try the same thing using the account of the user who actually needs to connect and it's a no-go. Outlook simply keeps prompting for logon credentials. Next I tried using my own account and that too failed. Testing with a couple of other accounts worked perfectly.
Interestingly enough, with my own account I've used IMAP on a MAC before (internally) without a problem and I'm not aware of anything that has changed which could affect IMAP on my account.
Checking the user settings in ADUC showed that all accounts have the same Exchange protocol settings. Specifically, IMAP is enabled. A check of the event logs on the server reveals no entries for the connection attempts, making this kind of difficult to debug.
Has anyone here encountered such a situation and, even more importantly, what caused it?