If you want to keep people of your server, but want them to access their pop mail on the same box set up the following line in the password file:
Then create a script called
/bin/nologin that contains:
echo "Sorry, but your account is only allowed POP access to this host."
when a person attempts to log into their defined shell it will run the /bin/noling script, display
the message, and exit them from the system