How can you setup the Mandrill Return Path Domain?
If you'd like to configure a custom Return Path Domain, you'll need to first add the domain, i.e. track.example.com, in your Mandrill account under Settings > Domains > Tracking & Return Path Domains:
Once you've done that, you'll need to enter that domain in your MageMail account under Settings > Advanced > Mandrill Return Path Domain:
Once you've done that, if you send yourself a test email, for example in Gmail, you should see the domain show up in the mailed-by field: