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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?

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I don't have any suggestions other than to turn diagnostic logging for the IMAP4Svc component to maximum on the properties of the Exchange server. This should give you enough logging to see what's going on. Just remember to turn diagnostic logging off when you're done troubleshooting the issue. – joeqwerty Mar 5 '11 at 13:40
up vote 1 down vote accepted

After turning on debug logging, as per joeqwerty's suggestion, I found that the accounts which were unable to connect using IMAP were throwing error 0x80040920. A bit of Googling located this Microsoft KB article.

Although the article talks about POP3 it is also relevant to IMAP. Specifically, the email alias set on the affected accounts did not match the logon ID. The solution was to simply change the alias to match the logon ID. I also tried deleting the alias but that didn't work.

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Well dang it. I was going to post that in an answer but thought it might be better to turn up the diagnostic logging first. – joeqwerty Mar 8 '11 at 21:56
@joeqwerty, it was the diagnostic logging that led me to the answer. Thanks for the suggestion. – John Gardeniers Mar 8 '11 at 22:26
Glad to help... – joeqwerty Mar 9 '11 at 1:16

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