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DKIM alignment
Problem reported by Samuel - 12/6/2025 at 6:29 PM
Submitted
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How do you configure DKIM alignment with two different domains?  For example, SmarterMail is configured for DomainOne, and I want to configure SmarterMail to send as DomainTwo as well with both being configured with DKIM.  DomainTwo is a client I'm trying to send emails on behalf of.
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Douglas Foster Replied
12/6/2025 at 8:13 PM
I assume you want the SMTP Mail From.address to be Domain1 so that you receive bounce messages.   SmarterMail is not optimized for this role, but I think you can kluge it.

Configure both domains in SM.   Configure DKIM on domain2 and have your client out publish the public key in domain2 DNS.

Send from domain2 to a mailing list in domain1.   Ensure that the mailing list does not modify subject, body, or From address, but does rewrite the SMTP MailFrom address (which is never signed)

Test to prove that message arrives to a test gmail account with a verifiable signature
   
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Samuel Replied
12/7/2025 at 5:25 PM
Some clarification, I would like to use Domain1 to send emails with the From.address of Domain2.  I own Domain1 and my customer who owns Domain2 would like me to send out emails on their behalf from their domain but with DKIM alignment.  Does that change anything with the recommendation above?
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Douglas Foster Replied
12/7/2025 at 7:59 PM
No.

Your other option is to configure Domain2 as a normal domain on your server    Bounces will go to the client's MX.  The client configured your server in his SPF policy, and optínally publishes your DKIM private key as well.

Relaxed Alignment, which is what is usually selected, means that a verified identifier (DKIM or SPF)  is in the same organization as the From address.  Strict alignment means that they must match exactly.  The choice is made in the DMARC policy.   If subdomains are not being used, there is no distinction.

Domain1 is never aligned  with domain2.   He needs to use his SPF or DKIm entries to prove that you are acting at his request 
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