I have two domains setup with the Google app. Domain A is the primary domain, and domain B is a domain alias to A. Both domains have valid DKIM keys generated, added to the domains DNS as valid TXT entries, and Google apps shows both as 'Authenticating Email' in the Admin console.
When sending email from domain A it has the correct 'DKIM-Signature' header for domain A and shows as being signed in Google etc. However when I send email from Domain B it has the 'From' header as Domain B but the 'DKIM-Signature', 'Sender' and 'Return-Path' headers from Domain A. This is resulting in mail from Domain B failing the DKIM check.
I have set up the domain B email as a 'Send mail as' address under Settings -> Accounts, and tried both using the default 'Send through ' as well as trying 'Send Through SMTP Servers' using smtp.gmail.com to no avail.
Any ideas on how to make this to work?