When you login to the system, you are informed by the system if you have mail. You can read your mail by simply typing mail. The mail program will display a numbered list of new messages with their subject headers. To read a message type n where n is the number of the message you want to read. To delete the message numbered n type d n. To save a message in a file use the s command. To exit out of the mail and save all messages, without deleting, in the file mbox in your home directory, use q. To reply to a mail use the Reply or R command. There is also a help command in the mail program.
If you want to send mail to any user then you need to type
mail <user_name>. It will then prompt you for a subject of the
mail message. You can leave it blank if you wish. After you have
finished typing the message, press Ctrl-d to send it. While
typing the message you can only edit the current line. However you
can invoke the vi editor any time by typing 
v
on a line by itself. This puts your current in a
temporary file and allows you to edit the message using your default
editor. After you are
finished typing, exit vi and press Ctrl-d ends the message.
A mail program with a convenient interface is mutt. It is available on all Linux systems and, currently, mutt is the favorite mail reader of the authors. Other common choices are pine and elm.