Your software
My Mautic version is: 5.2.7
My PHP version is: 8.2.29
My Database type and version is: MariaDB 10.11.13-MariaDB-0ubuntu0.24.04.1
Your problem
My problem is: when I send an email from my own SMTP with my hands via Postfix, the email is sent with DKIM, but when I do the same thing, but via Mautic, for some reason the email is sent without a signature.
on the pic left - Mautic, right - hands
DKIM signing happens on the SMTP server where you submit the mails.
DKIM Signature has to be performed at the last step before sending, so at the MTA level…
When you are using Postfix, you are also using rSpamd or Opendkim to sign the message. But Mautic is not an MTA, so it does not sign messages, which is absolutely normal.
The best would be to push messages to Postfix, so the MTA will handle everything, queue management, delivery and DKIM signing.
