SMTP port not listened

[Peter Mason]

We're trying to use the default of MMDF with SCO UNIX, and our machines
are ethernetted. Seems that we can happily use mail locally, and that a 
mail is able to be sent out to a non-SCO UNIX system, but the SCO UNIX
machines are unable to receive mail via TCP/IP. If the SCO UNIX machine does a
"netstat" then we see nothing listening to the SMTP (port 25). Mail gets
bounced after 3 days with a "Connection refused" message.

Which process in the SCO system is supposed to be monitoring this port,
and how do we get it started up?