Very Computer

1. transition from isp 'A' to isp 'B' routing problem

I am switching from isp "a" to isp "b".  Each one has it's own
dedicated router on my DMZ. I have one server (email) with one nic
card on the DMZ.  I also have a third dedicated router on the DMZ
which goes to my private network.  Isp "b" is the one I am switching
to and want everything to go through.  Routing all internet traffic
from the private network to isp "b" was simple.

        isp "a"         Isp "b"
       -----------     -----------
       | router  |     | router  |
       -----------     -----------
            |               |              ----------
             ------------------------------| router | --> private
network
                     |                     ----------
                ---------------
                | mail server |
                ---------------      

   The problem is going to be the transition period when email is
coming in through both isp's when some servers are sending to the old
ip and some to the new ip before everything is updated on the
internet.
   Currently the internet routes all my mail through isp "a" to my
dedicated ip address on the mail server and my default route on the
server is through isp "a". This works fine.  If I change my default
route to go through isp "b" I stop getting mail.  Apparently you
cannot come in through one router and return through a different one.
It seem I can only communicate with the internet through the router
which is the default route.
   By the way router "b" also has a dedicated ip but does address
translation to the DMZ network which is probably the problem.
   What I need is a way to route traffic that come from router "a"
back to router "a" and traffic from router "b" back to router "b".
   I have read many howto's and posts but none seem to address this
issue.  I was wondering if something like the following would work:
    ip route add default via router-A-ip nexthop via router-B-ip
I found a couple of posts which refered to this.  Also where can I
find a complete list of comands for ip?

   Thanks in advance for any help or suggestions you may have.

Joe D.

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