Depending on how you use your system, you might want to prevent users from accessing e-mail through SMTP and POP servers. You can prevent e-mail access entirely or allow access occasionally.
Preventing Simple Mail Transfer Protocol access If you do not want anyone to use Simple Mail Transfer Protocol (SMTP) to distribute mail to or from your system, you should prevent the SMTP server from running.
Preventing Post Office Protocol access If you do not want anyone to use Post Office Protocol (POP) to access your system, you should prevent the POP server from running.