If he is concerned with offloading the emails because he wants to avoid strain on his email system, then I would strongly suspect that the people who he is trying to send emails for are from companies that his email server doesn't normally forward emails for.

Therefore he would be forging emails for domains that are unassociated with his server, and would be flagged as spam.

If the emails are forged for his own domain, then externally this issue goes away. I know why this is useful, and I have forged emails myself. However he still may have an unusual characteristic which causes his email to be flagged by some as a sign of spam. (Been there, done that as well...)