Emails not DKIM signed when email address domain differs from the mail server's domain

1

My setup:

Server 1 (CPanel):
Domain: domain.tld
MX: mail.domain.tld
Email: [email protected]

Server 2 (HestiaCP 1.7.2):
Domain: sub.domain.tld
MX: mail.sub.domain.tld

I have added server 2’s SPF and DKIM records to server 1’s DNS zone, however if I want to use Server 2’s mail server (mail.sub.domain.tld) to send emails using the [email protected] email address the header does not have a DKIM signature.

I’m not sure if this is configured intentionally on Hestia CP, but before when Server 2 used CPanel it worked and I could send DKIM signed emails using the [email protected] email address from the mail.sub.domain.tld mail server.

Can I make any changes to exim to DKIM sign emails sent from [email protected] using the mail.sub.domain.tld mail server?

2

Use SMTP relay otherwise it is not possible

3

Thanks for the reply.

Yes, for a test I hardcoded the exim config file to use the mail.sub.domain.tld dkim.pem file to sign the [email protected] email and it failed the dkim test which is worse than not having a dkim signature.

I restored the exim config to the default one and added the Amazon SES SMTP credentials to the SMTP Relay field for the sub.domain.tld domain, but emails are still sent using the local mail server and not via Amazon SES. I tested the Amazon SES SMTP and it works, but am I doing somthing wrong with the SMTP Relay setup?