A user is prompted for user name and password intermittently while logged into the domain.
Usually this happens when changing the AD password and it does not sync up with Exchange. In this instance changing the password again and restarting did not work.
I went into Control Panel > Credentials Manager and deleted the saved Outlook Generic Credential and then recreated the profile to solve the problem.
After deleting and recreating a user’s mailbox certain internal user’s were faced with error #550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found ## when attempting to send mail to the user.
The reason and solution as to why this error was received is in this article
https://www.simple-talk.com/content/print.aspx?article=628
I needed to add a X.500 email address to the recipients mailbox in order for message to get to his mailbox.
The X.500 address of a message sender can be retrieved using a tool such as Microsoft Exchange Server MAPI Editor to open a message and get the PR_SENDER_EMAIL ADDRESS property:
The addess is added using these steps in Exchange Management Console
Frank's Tech Blog 