Electronic mail is usually the most important method for communicating via computer. An electronic letter is stored in a file using a special format, and special mail programs are used to send and read the letters.
Each user has an incoming mailbox (a file in the special format), where all new mail is stored. When someone sends a mail, the mail program locates the receiver's mailbox and appends the letter to the mailbox file. If the receiver's mailbox is in an another machine, the letter is sent to the other machine, which delivers it to the mailbox as it best sees fit.
The mail system consists of many programs. The delivery of mail to local or remote mailboxes is done by one program (e.g., sendmail or smail), while the programs users use are many and varied (e.g., Pine or elm). The mailboxes are usually stored in /var/spool/mail.