Redundant ADSL Connection and Additional MX Records.

Hi.. i have a SME 5.5 U6 with portforwaring and email server on Lan. (other PC not SME)

I used the portforwarding RPM and use it on port 25,110 and 143. This work Fine!.

I have a new and additionl ADSL connection and I wish setup this connection like additional MX record for domain.

I Try port forwarding in a new SME server to the same email server on lan but it not response. I add an aditional gateway in the email server on lan, point to ip of second SME server but y suspect that the reply from the second conection is route throu first conection.

Diagram
------------

ADSL 1 -> SME 1 ->Port Forwarding -> LAN UNIQUE Email Server------ This is OK!

ADSL 2 -> SME 2 ->Port Forwarding -> LAN UNIQUE Email Server --- This Fail!

Any body can help-me or sugest one alternative to this configuration ??

Thank in advance and sorry by the english

Luis