Making users to use port 587 for submission emails

[Sasa Stupar <sasa@xxxxxxxxxxxxxxxxxxxx>]

Hi!

I am running Sendmail 8.13.4 on FC4 with cyrus2. I have configured sendmail 
to listen on ports 25 and 587. Since all of the client mail software use 
port 25 by default instead of 587, how do I make users to use 587 and deny 
access to 25? Something like: when they try to connect to port 25, they get 
back error - you are not a server so you must use port 587.
Any idea?

Regards,
Sasa