pgus, yes you can do this quite easily.

We run several domains through SPI and for those hosted internally we just list the domain name in the Local Domains area and rely on SFI to relay mail to the Destination Server specified in the Configuration tab.

For those customers running their own mailservers (such as Exchange etc) we simply specify the destination server.

Example:

customer1.net
customer2.com:xxx.xxx.xxx.xxx
customer3.org:xxx.xxx.xxx.xxx:4425

In the above example customer1.net mail is sent to our internal mail servers (IP set in Configuration), customer2.com mail is sent to xxx.xxx.xxx.xxx on port 25 and customer3.org mail is sent to xxx.xxx.xxx.xxx on port 4425 (they may not want to use port 25)

Hope this answers your question.