Hope I can explain this properly! Is it possible to port forward from two different internet connections to one machine? Basically, I am about to change my ADSL line. I have my domain mx settings to my mail server on the current one. I will get the new connection which will have a different IP address. I assume there will be some crossover while my new mx settings are propagating, therefore I want both connections pointing to the same machine. I assume this will be possible?

It's not that simple. Although you can give both routers a port-forward to the same machine, that server can only really have one default gateway.

If you know the originating external IP then you can set a static return path, so for example you could set a VPN to work over one route and browsing to go over the other, but in your case I guess you won't know where the traffic is going to come from?

The only thing you can try is adding a second default gateway, but it will cause a lot of timeout issues.

Alternatively, if you have a router with two WAN ports then that should work because the router will know about the NAT sessions. This is the only good way to do it.

Cheers, I hadn't thought of the default gateway, doh!

It is running os x server and that can have two ethernet connections I think, but how to set that up and how it works, I don't know.

Two Ethernet connections might not help.

Your basic problem is that the server needs to make a mental note of where the connection came in, and to send the reply down the same way. A load balancing router is pretty much designed for the job, but I'm afraid I don't know much about OS X and what it can do. Maybe there's an app for that.

Well I think each connection has its own ip address and possibly gateway. I'll need to dig around.

Are you changing ADSL providers on the smae line or is it a new line for the new connection?

Either way it's fairly easy to do.

New line.

Here's what I would do.

Leave the existing A record in place.
Create a new A record (mail2.domain for example) going to the new IP
Add that A record as a 2nd MX record with a priority of 50 (any number as long as it's bigger than your current record)

If you've got a new router then configure it up with the same LAN IP as the old one (& relevant static PAT entries). If reusing the old router you'll have to wait until the new line is active.

When the new line is active simply unplug the old router, plug the new one (connected to the new line) in it's place. Test it then modify your existing A record to point to the new IP.

Job done with no lost email. You'll just need to give people who access mail through imap or pop services etc the new details (or the new webmail address).
Remember to get rDNS set on the new line asap

Brilliant, thanks for that.