We use SmarterMail as our primary mail server for our SaaS marketing system. Our customers send thousands of emails through our service using their own domain as the FROM address. We SMTP into the server and send using our domain, however, (I believe because the FROM does not match the domain), the server does not DKIM sign the emails.
Is there some way we can get this to happen?
Note that it is not an option to add each of these domains as a domain alias, as this changes the behaviour of how emails to these domains are handled by SM, and also the administration of this would be very impactful.