What if instead of using two different ip nat inside statements with two different external IPs, you create a single external address in your NAT statement.
I assume from your post that you have two interfaces with IPs of 1.2.3.4 and 1.2.3.6. Could you create a NAT statement like this:
ip nat inside source static tcp 192.168.1.2 25 1.2.3.100 25
I think that what this would do is to forward any traffic for SMTP to the 192.168.1.25 address. Upon replying, this inside host traffic would be translated back to 1.2.3.100.
So, no matter which outside interface the reply goes out of, it would send the traffic with a destination address of 1.2.3.100.
You don't have to use IPs associated with a physical interface in NAT. You just have to own the IP that you are using in the NAT translation. And, routers would need to know how to get to the IP (1.2.3.100 in this case).
I think that what is happening in your case is that if you telnet to 1.2.3.4:25 it is hitting that interface but the reply can be translated with an address 1.2.3.6 and this just won't work because the host is expecting a response from 1.2.3.4.
Let me know your thoughts.
