Hi Ed, 

It's possible that sender A has their Default From Address set to a different address. If that's the case, when they log into SmarterMail, their email compose window would show the Send From address as Person C, as seen in my screenshot below. When Person B receives the email from Person A, they'd see the Person C email address as well. 

To verify, have Person A log into SmarterMail and click on the Settings icon. In Account settings, check the Default From Address. If it matches Person C's address, they'll just need to switch that back to their account. 

If Person A's Default From Address is set to their own account, and if they are not manually changing the From Address on each message they send, please help me troubleshoot this further by answering the questions below: 

Thank you! 

Actually, the default from address was not always configured.

However, the problem went away as we migrated away from SmarterMail. 

Thank you for the response.