You mailing return address is based on 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://web.dealmachineapp.com with your computer.
Navigate to Campaigns
Press the "Manage Templates" button
Press the "Manage Signatures" button
Select the signature you wish to edit
Edit the return address information to your liking
Click "Update Signature"
That's it! Now all mail templates using that signature will reflect those changes!