To answer your question, you can create an account / email address through the command line on a Linux / Unix system.
The first step is to create a user account in this system for this user. There are different ways to do this. I found this guide, The Ultimate Guide for Creating Users on Linux / Unix , to be one of the best reviews.
The next step requires understanding that email on Linux systems is often handled by two different systems / commands.
An email is sent first. This is done quite often through sendmail. If the user can execute sendmail, he can send mail via smtp to the world. This is too simplistic, but provides enough to get you started.
Receiving mail in the system and delivering it to users depends on the mail system used and the protocols used. If you are using postfix, the following article shows how to set up an email account .
My answer assumes that you are not using LDAP or another directory service to support users.
This question probably refers to serverfault . I could not find an exact match there, so I answered this and noted that it was moving.