Server :: How To Change Postfix Return-path

I just set up an internal mail server for my office using Postfix, Dovecot and Squirrelmail. It works great except for one tiny problem. When I send mail, I address it to user@myoffice (no .com, .net, etc) and it works perfectly. However, somewhere in my work, I screwed up and for whatever dumb reason put myoffice.net somewhere in there and now the return-path always shows up as user@myoffice.net instead of just user@myoffice. Does anyone know how to fix the return-path so that when someone clicks reply it goes to user@myoffice? I've Googled and searched here without finding my answer, apologies if it's out there and I've just failed to see it. Rest assured I have done a couple hours of searching before finally asking for help.