Hi John,
There isn't any SMTP forwarding capability of this type in the e-smith server and gateway yet. How big is your site? (Number of computers, email acocunts, etc.?) At some point it makes sense to consider getting a commercial firewall/gateway solution. Then you could set up the e-smith server in server-only mode, and use its ftp, web, and other servers. Then you could easily continue to use your existing mail server as well.
In future versions of the e-smith server, we are considering adding port forwarding capabilities which would solve your problem.
Best regards,
- Joe