Transitioning SMTP with Firewall between ISPs

["Michael Erdely" <mike@erdelynet.com>]

I have an OpenBSD 3.0-stable firewall running PF.  Behind the firewall,
running on 10.0.0.0/24, I have my private network with a mail server.

We are in the process of switching ISPs, so our public IP Addresses are
going to change.

Both public circuits terminate at a hub.  The public interface of the
firewall is also plugged into the hub.
My default route on my firewall is the router for our old ISP.  I can
connect to our old IP Address/Port 25 for email fine.
For a smooth transition, I planned to add the new IP Addresses as
aliases on the public interface on the firewall and rdr the traffic back
in to the SMTP server, just like I am for the old addresses.  But, it
seems that the firewall doesn't know how to route the traffic back out
to the new router even though the traffic originated from that router.

So, how would I make it so that my public interface would successfully
respond with both IP Addresses during the switchover?

Thanks,
-ME