Some customers complained that the Outlook delegation isn't function as it should. Some of them reported on NDR errors (e.g. “The e-mail address could not be found. Perhaps the recipient moved to a different e-mail    organization, or there was a mistake in the address. Check the address and try again. <ServerName.DomainName.local#5.1.7>”) while others reported freezing Outlook etc.

Mr. Chris Berry wrote a post that cover “Deleting Old Delegates using MFCMAPI” process. Using this guide you to delete orphaned/mismatch settings in the user mailbox.

For further information, please review:

Deleting Old Delegates using MFCMAPI