Asynchronous messages sent over the internet. An E-mail is transferred from the sender to the sender’s Mail Server, then communicated to the receiver’s Mail Server using Simple Mail Transfer Protocol (SMTP) and finally to the receiver.
How a user communicates to its Mail Server varies on the mail client being used. Microsoft Outlook uses Internet Mail Access Protocol (IMAP).
Format
E-mails consist of header lines and the body of the messages separated by a blank line (CRLF).
Every header must have a From: header line and a To: header line. It may include a Subject: header line and other optional header lines.
E-mail header lines are different from SMTP headers. They define the message while SMTP headers define the communication between mail servers.
Examples
A typical message header:
From: alice@crepes.fr
To: bob@hamburger.edu
Subject: Searching for the meaning of life.