Perhaps an issue with a signature or quote being mis-recognized. Maybe there are b dashes in the email that look like the end of the email to the scanner.
There is not.. Discourse does not reveal user’s email addresses.
I once wrote a plugin that included the email address if the sender in the message for this reason. I don’t remember the name and the client doesn’t update discourse often, so it likely is broken how. If you have a budget and can install plugins, you can send me a pm.