3rd party delivery via our Brightmail messaging gateway.
We wish to have all mail sent for our domain DKIM signed. We have a marketing vendor who sends on our behalf who is unable to implement DKIM.
What is the best way to use our SMG to relay mail sent from them. e.g. They send to our SMG, which then does final delivery.
The most obvious (but not secure) would be to expose our outbound interface to the vendor and add their IP address under outbound mail acceptance.
Is there a way to do this (MTA to MTA) using SMTP Auth?