How to locate where mail address is being used in SharePoint 2010

Over the past few months, a number of people have retired from the company. Now mail messages being generated within SharePoint are bouncing back to the admin mailbox. The addresses which used to be in Active Directory have been removed, and Exchange is returning a message saying that mail for the user@site.org could not be delivered - "NotAuthorized".

Unfortunately, there is nothing in the mail header that indicates from what part of the farm the mail is being issued.

The subject line is vague and attempts to search for the subject returns too many document hits and nothing that appears to be related to the actual message.

Does anyone have any times, or sample powershell scripts, they have used to solve a similar issue?

Thank you