2 ISP connections to single Exchange 2003 server

Gang,
I was reading this Experts Exchange discussion, but it seem to stop without a solution.  https://www.experts-exchange.com/questions/23652696/Need-to-connect-Exchange-Server-to-multiple-ISPs.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.

Thanks
Lee