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Fixed: Outlook 2010 - 550: No Such User Here

Today I had a client call and say they were unable to send email through our mail server. The error message they received was:

From: System Administrator
Sent: Friday, January 20, 2012 1:48 PM
To: xxx xxxx
Subject: Undeliverable:

Your message did not reach some or all of the intended recipients.

      Subject:  FW:

      Sent:     1/20/2012 1:48 PM

The following recipient(s) cannot be reached:

      'xxx xxxx' on 1/20/2012 1:48 PM

            550  No such user here

My first thought was to have them check their authentication settings, as our mail server requires authentication to send. The authentication settings were not enabled, so I had him set the 'My server requires authentication' to on and I had him set the 'Use same settings as my incoming server.' option.

After saving the settings, he tried to send another message and received the same error. WTF? All of his settings were correct, why wasn't he able to send?

After scouring the internet, I found nothing that would help. I was going to resort to setting his Outlook to use his ISP's outgoing server. Before doing so, I had him change his outgoing authentication from 'Use same settings as my incoming server' to 'Logon using'. Then he re-entered his username and password (which are the same as his incoming). Wouldn't you know it, he sent a message successfully!

I'm not sure if this is a bug with Outlook 2010, but if you ever run into the error, 550: No such user here, try changing the authentication method and force the user to re-type their logon credentials.