Solution by Event Log Doctor

We got this error when connecting to the Imap4 component on an Exchange server using an IMAP4 client such as Entourage or Evolution. Even though the username and password were specified correctly, we were unable to log in.

This problem occurs when the Exchange Alias does not match the username. For example, if the Exchange Alias is "FrankSmith" but the username is "fsmith".

To resolve this problem, make sure the Exchange Alias matches the username, using "Active Directory Users And Computers".