[expert] Listening port for Sendmail

[R_Yeo]

Hi,
        At work, we have a Class C address with it's own mail server. 
My Linux box is configured with sendmail to reply to mail addressed to
me via my ISP.  Corporate IT is clamping down on smtp servers due to 
spammers.  Is there anyway to close down my smtp port (25) so that it
will not be listening or only listening to the local host?  Corporate
IT does a periodic scan of the port to look out for unauthorised
servers.
        OTOH, if someone has a suggestion for qmail or postfix, I'm all
ears.
-- Ronald