2 ISP connections to single Exchange 2003 server

I was reading this Experts Exchange discussion, but it seem to stop without a solution.  http://www.experts-exchange.com/Software/Server_Software/Email_Servers/Exchange/Q_23652696.html?sfQueryTermInfo=1+exchang+isp+multipl

We are changing our ISP over which we send/receive email.  During the DNS propagation time I would like to receive via our other ISP, but still send email originating in our office out the new ISP SMTP server.  I figured all I needed to do was set up a second MX record.

But in testing if I telnet second_ip_address 25 from outside I get no response.  Doing this internally within the lan works fine.  

Can I get it to respond to email from both routers?  I could just change the default gateway for a couple of days, if necessary, but I would like it for failover purposes.

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MesthaConnect With a Mentor Commented:
The default gateway is the key here.
You can only have one default gateway for the traffic to go back out through.
You would need to have a second machine for both gateways to be active, so the email can be delivered in to either server.

What I do for ISP change is use a dynamic DNS account. The dynamic DNS host is added to the MX records at least 48 hours before I make the change, so that it is fully propagated. Then when it comes to the switch over, I simply change the dynamic DNS IP address. That change is live in a few minutes. The real DNS record can then be changed. You are without email for a few minutes at most.

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