DNS and Backup Mail Server

[Drew Tomlinson]

While my FBSD system that performs mail services for my home network was
down for a few days, it occurred to my that having a backup server would be
optimal.  Since I am running FBSD as a firewall/gateway, I installed Postfix
and configured it as a backup mail server per the docs.  However when I
test, mail never makes it to the backup server.  I assume this is because I
don't have proper DNS and/or NAT entries.

Here is a diagram of my home network:

                  ISP
                   |
                   | Public DHCP address
                   |
           3Com ADSL Modem/Router
           (Router performs NAT)
                   | (192.168.10.1)
                   |
                   |
                   | (ed1 192.168.10.2)
            FBSD Gateway (Blacksheep)
                   | (ed0 192.168.1.2)
                   |
                   |
              Internal LAN
                   |
                   | (192.168.1.4)
     FBSD Primary Mail Server (Blacklamb)

My 3Com DSL Router *has* to be a router to work with my ISP and thus, it
*has* to perform NAT to pass traffic from my network to my ISP.  Therefore I
do not perform NAT on the gateway as it's redundant to do NAT twice.  Any
traffic that is not defined in the NAT is forwarded to the outside port on
my gateway (ed1 192.168.10.2).

To get mail to my primary mail server, I have a static entry in the NAT on
the 3Com router to forward all traffic on port 25 to 192.168.1.4.  I use
ZoneEdit for DNS services and have an 'blacklamb.mykitchentable.net ->
public IP' MX record.

So with my limited understanding of DNS, it seems there is no way to have
mail failover automatically, correct?  However if Blacklamb (192.168.1.4) is
unavailable, I should be able to edit the NAT entry so traffic on port 25 is
forwarded to Blacksheep (192.168.10.2), right?  Would there be any reason to
change my MX record?  I don't think I would need to change it as my public
IP remains the same but I am unsure.

Any insight or nudges to links for beginners on this issue would be
appreciated.

Thanks,

Drew