Here's the scenario. You host your primary mail server at an ISP that allows SMTP (port 25) into their network. You host your backup mail server at a different ISP that also allows SMTP into their network, but blocks SMTP from leaving their network (to stop spam zombies). Here's the problem with that. When the backup mail server relay's mail to the primary mail server (who's listening on port 25) it will be blocked.
To fix this we will need a way to get the backup mail server to send it's mail to the primary mail server on a higher unblocked port. This means we have to get the primary mail server to listen on it's regular SMTP port and a new higher port.
This example will use Postfix to do this. This test was done with Postfix version 2.5. I'll assume it will work with higher versions.
We will start with the primary mail server. All we need to do is get it to listen on a second port. To do this put the following line in the /etc/postfix/master.cf file. You can put this line under the line that starts with smtpd if you like. It will make Postfix listen on port 2525. Postfix will still listen on port 25 as usual.
2525 inet n - - - - smtpd
Then restart postfix
Now the primary mail server is listening on port 25 and port 2525. On to the backup mail server.
On the backup mail server edit the /etc/postfix/transport file. Put in the line below. Substitute pantz.org for the name of the domain you want to receive mail on. Change mail.pantz.org to the name of your primary mail server.
Now make the transport db.
That's it. No restarting postfix needed. Now when mail is delivered to the backup mail server for the pantz.org domain it will be sent to the primary mail server on port 2525 which the backup servers ISP allows out.
Don't forget to open any new ports on those firewalls since 2525 is an odd port. It's likely to be blocked.