According the our fine manual ...
If you want to forward e-mail to another mail server for processing, enter the mail server IP address in the box marked Delegate mail server . A common use for this is if your server is receiving inbound e-mail from the Internet, but you would like to pass that mail to a different mail server on your internal network. Note: Delegate mail server implies that all mail which is accepted is passed on to the delegate mail server (IOW, that other guy is the mail server, I'm not, so I expect him to do everything, eg spam filtering) If you intend to have an external mail server handle mail for your domain, just send the mail directly to that mail server, via the MX record for your domain.
My SME Server is set up in server/gateway mode. I would like a separate computer in my local network to handle traffic (both web and mail) for a single domain while having the SME server handle everything for the other 2-3 domains.
For web traffic, I can setup proxypass or some other pass-thru vehicle in Apache. But what is the best way to get the email for only one domain passed along to the other machine?
Thanks for your help.
John