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Microsoft Outlook use Windows Logon credentials to authenticate against domain to access Exchange server. If it keeps popping up to you for logon credentials, then potentially something may not happening between you workstation and the domain controller (DC). Here is what you should check:

Message submitted to Categorizer, what next?
There are two scenarios that applicable to this, one is incoming and one is outgoing email.
Incoming email
Sometimes when you use Exchange System Manager’s Message Tracking, the result appears to be puzzled.