You mailing return address is based on your account signature.
To edit the return address, or anything at all on your signature, follow these steps:
Open your DealMachine mobile app or log on to https://dealmachine.com with your computer.
Navigate to Mail > Signatures.
Select the signature you wish to edit (Multiple Signatures is an Enterprise Exclusive, you get one with the Basic Plan)
Edit the return address information to your liking.
Click Update Signature or Tap the Check Mark on the Upper Right Corner of your Screen.
That's it! Now all mail templates using that signature will reflect those changes!
Note: If your signature isn't updating on your postcards, try to hit "Reload Preview" below the mail preview to refresh it and see your updated information.