When adding a domain alias there's an option to Verify MX record in DNS before add.
This always fails on my installation as the server is behind a NAT firewall and it doesn't have the external IP assigned to it. The server only has the internal IP (10.0.0.x).
I may be being a bit a slow, but I don't think there's a workaround for this other than to not verify the MX record before it's added?
Not verifying the MX obviously runs the risk of someone accidentally adding 'gmail.com' as an alias and getting all internal server email that's intended for gmail.
If there's not a workaround that I've overlooked, could it work where this MX check also tests against the SmarterMail Hostname as set in the Admin General Settings for the server?
The MX record should match this hostname, and if it does it's safe to assume the domain passes the MX record check.