Outlook periodically/frequently prompts for login credentials after migration to new mail server

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http://serverfault.com – We are experiencing a problem after migrating to a new mail server where users that connect to the mail server through Outlook (using IMAP) get prompted periodically/frequently for their username and password. Pop-up prompts "Enter your username and password for the following server" and contains the name of the server, username and password. Since most users have opted for Outlook to save their passwords, they simply have to click on "Ok" to log in successfully and continue, but after a few minutes they will be prompted yet again. This probably happens when Outlook tries to connect to serve (HowTos)