smtp port forwarding

[#1]

Hi
I use a static ip and have set up port forwarding for smtp (port25) to a device on my LAN. In my DNS I have set up a secondary MX which is a second machine on my LAN.
I have logged in to the Thompson router and created a second smtp port forwarding mechanism and directed that to the second device on my LAN; so, i now have 2 port 25 forwarding instances on the router each pointing to a different device on my LAN.
The reason for this is because should my primary mx mail server be unavailable for whatever reason, the router will then forward smtp traffic to the second device.
It has allowed me to do it and is showing in the Games & Applications section when logged in to the router but i'm wondering if this router supports what i'm trying to achieve. I'm wondering that just because it's allowed me to do it and it's showing up there, will it actually behave as I want it to should my main machine be down.
Does anyone know if this is supported on the Thompson routers that Plusnet provide?

[#2]

My suspicion is that it's allowed you to do it because it doesn't check/prevent it.
I also don't see how you expect this to work,  if both the primary and secondary entries in the domain point to the same IP address (your static ip), how is the router supposed to tell the difference when it only forwards based on port?

[#3]

You actually need to request a block of IP addresses and set up rules to forward by having different IPs for the incoming primary and secondary MX servers.
However what won't cover by that is what happens when your internet connection is down. I'd have thought a more sensible approach would be to find a third party that can provide a backup mail service elsewhere. There's some suggestions at the end of this: http://community.plus.net/autoturnmx-last-decommission-faq-july-2011/

[#4]

I wouldn't bother with a secondary MTA at all, particularly one that is co-located. If you can't fix a problem with the first within the time limit of mail being returned to the sender as undeliverable (several days at least) then you should probably think twice about running an MTA in the first place.
Of course, if it's just for a bit of fun and experimentation with such matters then, as the other have mentioned, you will need seperate public IP addresses for each MTA.
Mathew