How can I send a DKIM signed email from a domain alias in Gmail?

Question

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?

Answer 1

The domain alias is not the problem.
With Google the user account alias is the problem.

How Google choose to DKIM sign or not

For Google to add the DKIM signature the username must be one of the Google account aliases.

Say you have DomainA.com(primary) and DomainB.com(alias) Your google account is [email protected]

DKIM signature will be set when sending from [email protected] and [email protected]

But the signature will not be set when sending from [email protected] or [email protected] because ´other´ is not a user alias.

Solution: Add user alias

The solution is add the other as a user alias.

1. Login to admin.google.com
2. Go to Directory > Users and click your user
3. Click the first block: User Information
4. Skip Contact information and open settings for the second: Alternative email addresses (email aliases)
5. Add other as an alias to your primary domain
6. Click save

Now you will get DKIM signed emails from other@... for both your primary domain and all your aliases.

Answer 2

From your admin console go to domains. Then remove the domain as an alias, add it back as a secondary domain. Then you will have the ability to get the DKIM key for that domain.