smtp forwarding

[Eitarou Kamo]

Hi,
I'm new to this list.
I have some questions. I would appreciate any suggestion.

I'm using freebsd4.7 and 2 qmail server as the smtp server on it.
Needless to say, 2 qmail server are duplicate server primary and secondary.

Now then, I'm also using ipfw2. I would like to forward the smtp packet
to the
current(alive) mail server with priority. I mean that I'd like to
forward the smtp
packet to the secondary server when the primary server is down or panic.
Is it possible? I'm wondering if ipfw can know the status of the mail
server
(dead or alive) in advance. And if possible, I expect the solution of
the ipfw rules.

Assumption:
mail domain is "@hoge.com(global)"
primary mail server address is "192.168.100.4 (private)"
secondary mail address is "192.168.100.5(private)"
ipfw interface is "fxp0"


Thanks in advance.

Eitarou