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All of the sudden IMAP4 authentication started to fail for my e-mail account. It was working just fine for years, and yesterday I started getting authentication errors.

Our organization runs Exchange 2010 hosted on Windows Server 2008 R2.

What's peculiar is that I can successfully login using OWA-interface from browser, and also I can set-up a POP3 connection from any e-mail client - all that using the same username and password. It's only IMAP4 that fails.

What's even more peculiar is that in the whole organization it's only me who has these problems with IMAP4. For instance, here're two connection logs, one is from test account and another is mine:

- Connecting to IMAP server our.exchange.server.com on port 993
- Initiating TLS handshake
- Certificate S/N: our-certificate-sn, algorithm: RSA (2048 bits), issued from 7/16/2018 to 12/26/2019 12:00:00 PM, for 2 host(s): *.server.com, server.com.
- Owner: "*.server.com".
- Issuer: "US", "DigiCert Inc", "www.digicert.com", "RapidSSL RSA CA 2018". Valid from 11/6/2017 12:23:33 PM to 11/6/2022 12:23:33 PM.
- Root: "US", "DigiCert Inc", "www.digicert.com", "DigiCert Global Root CA". Valid from 11/10/2006 to 11/10/2024.
- TLS handshake complete
- Connected to IMAP server (our.exchange.server.com)
- The Microsoft Exchange IMAP4 service is ready.
- Authenticating (user: "test1", method: "MSN")...
- IMAP server authentication OK, server says "AUTHENTICATE completed."
- Connecting to IMAP server our.exchange.server.com on port 993
- Initiating TLS handshake
- Certificate S/N: our-certificate-sn, algorithm: RSA (2048 bits), issued from 7/16/2018 to 12/26/2019 12:00:00 PM, for 2 host(s): *.server.com, server.com.
- Owner: "*.server.com".
- Issuer: "US", "DigiCert Inc", "www.digicert.com", "RapidSSL RSA CA 2018". Valid from 11/6/2017 12:23:33 PM to 11/6/2022 12:23:33 PM.
- Root: "US", "DigiCert Inc", "www.digicert.com", "DigiCert Global Root CA". Valid from 11/10/2006 to 11/10/2024.
- TLS handshake complete
- Connected to IMAP server (our.exchange.server.com)
- The Microsoft Exchange IMAP4 service is ready.
- Authenticating (user: "myname", method: "MSN")...
- AUTHENTICATE failed.

I checked logs on server, and there is a record about exceeding maximum simultaneous connections:

The number of simultaneous connections made by user myname@server.com exceeded the maximum (16)

So I closed all the clients I have (closed the e-mail client application on desktop, removed e-mail account from the phone) and waited for one day. Then I tried to connect from a desktop client, but still got the same authentication error, and this time there were no logs about exceeding the maximum of simultaneous connections.

My only guess is that my account got "locked" somehow on the server, but where can I check such a thing specifically for IMAP4 connections? Because everything else (AD, OWA, POP3) works just fine with the same login and password.

Update/solution

Simply restarting the server helped - IMAP4 now works for my account.

I won't post it as an answer as the nature of the problem remains to be unknown.

  • 1
    Is IMAP enabled for your mailbox? – joeqwerty Sep 27 at 17:05
  • It is enabled, yes – retif Sep 27 at 17:35
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Check the application logs in the event viewer, have any errors in logs?

In addition, you can use Microsoft Remote Connectivity Analyzer tool to test IMAP4 connection status.

  • There are no errors in the log on server when I try to connect from my e-mail clients, on the contrary - Security log says: "An account was successfully logged on". And when I run the tool you linked with a test1 account that I mentioned, then it's all good, but when I run it with my login and password, then I get this error. – retif Sep 30 at 14:45

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