Setting USERS to Debian LINUX E-Mail Ser

Hello,

Looking for help for on e-mail server that is on Debian LINUX
I am the IT Support for my company with little knowledge of linux.
The LAN Administrator had resign from my company, just leaving
me the login and password for the e-mailLinux server.

Looking for help in the commands of Debian Linux :
e-mail server

Logging in to Debian Linux E-mail server

Adding a new user

Changing passwords

And logging out of it.

This would help me greatly on moving up,
Thanks for any Help you can give me.

Peter.