So here's the problem.
"Someguy" wants to transfer his domain and website to me so I can take care of it. But Someguy's primary email address is "firstname.lastname@example.org". If he transfers thatsite.com's DNS to me, the emails will no longer get to his provider.
I could just set the MX records for thatsite.com to point back to his email provider so he would keep getting his email, but then I couldn't have an email at thatsite.com.
What I need to do is configure Postfix so it sends only emails addressed to email@example.com back to his provider's mail servers.