Limiting Usage of SMTP server

[MountaiNet Tech Support]

I just finished setting up Qmail for our POP3 and SMTP servers.  How can I
set it up so that only people who are dialed up to us can use us for an
SMTP server?  We have had some problems with people on competitors using us
for an anonymous relay system.  I want it so only people with our IP
address can use this server.  I've heard some stuff about denying direct
telnet access to 25.....is this possible?