It's not my account, but apparently I have all the permissions that were possible to give me on the
It's not my account, but apparently I have all the permissions that were possible to give me on the account.

mydomain.com I can't have another MX record. So I route a mail from mail@mydomain.com to mail@secretdomain.com. Now if I want to answer that mail, is it possible to use mail@mydomain.com as the sending address, so no one sees the secretdomain.com address?mydomain.com is setup properly, sure.
