NATing to a machine situated over a VPN

As I understand you normally use the MX IP which is a public IP to get your emails when your normal web connection is available; when the connection fails as you are VPN tunnel I think you need to use internal IP of server, rather than the public IP; if this is the case, you can configure your DNS to resolve the mail server on private IP as well; this would ensure that the clients would try both the IP addresses.

Thank you.

You could try creating a second MX record with a higher prio. The lowest prio will be tried first. If it's succesful then it's delivered. If it's unsuccesful it will try the next IP.

I'm not completely sure I understood the question correctly, but I think this the solution you are looking for.

JG

I am aware that I can have a secondary MX record and I have that in place.  It's more so the idea of having an external address at our data centre NAT'ed from a machine down a VPN.  



Remote Site-------------------------------Data Centre
                                 VPN
Internal Server IP                                  External IP
192.168.1.1----------------------------------1.1.1.1


       

I don't quite understand why you would do that. Maybe I still don't quite understand your question....

I would think that you would have the external IP address for your mailserver as your 'primary' MX and the internal IP address (i.e. the address that would be able to reach over the tunnel) as your 'secondary 'MX.

This way when your server is unreachable over it's external IP it would fall back to it's secondary, the internal IP and go through the tunnel.