I service lots of small businesses and some of them do not have the capacity or ability to own static IP internet connection where I am based.
ISP are charging an arm and a leg for static IP connectivity.
As such, we rely on dynamic DNS WAN-IP mapping services to run services in local networks.
I am just thinking and considering if it's possible for Incoming Gateways configuration - in Domain Forward to be altered to either allow IP address, or Hostname of the local mail server.
This will create significant help to a lot of small business IT mail services. They can have an external Email Server (SmarterMail) for relaying purposes, with antispam and antivirus, and an in-house smartermail server for their mailboxes in their AD or File server in-house since the load for emails would be significantly lesser.