On Fri, 15 Jan 2016 07:33:10 +0000, Paul Goyette wrote: > > Actually, I have a good reason for using a backup-MX - the primary > mail server is only reachable via IPv6. It is "hidden" behind an > IPv4-only NAT box. Connectivity between the primary and backup MX > machines is via an openvpn tunnel, running IPv6-over-IPv4 (the IPv6 > address range is globally visibile and routable). Do you actually need an MTA on the backup server? This could run a proxy/port-forwarder/load-balancer service to redirect the SMTP session to the primary, leaving you with just one MTA to manage. Best regards, Niall O'Reilly