Part 2: POP3 Email Server
As you already know, the SMTP servers consider an email message as delivered the very moment the message has been
added to the recipient's mailbox (see SMTP email delivery for details).
In the simplest (and most widely used) implementations the mailbox is nothing but a text file, named after the
recipient's email id (for example 'john' for 'john@company.com'). When a message arrives, the Mail Transfer Agent
simply appends it to the bottom of the file.
Please note that the mailbox file normally is being located on the same computer with the Mail Transfer Agent
(SMTP server) serving the recipient's domain incoming mail (it is worth to note that the term
"email account" generally refers to this mailbox file).
At the same time most email users
employ PC based software such as Outlook Express in order to receive their messages.
Obviously there must be some technique allowing the email client to retrieve messages from
the remote mailbox. And the name of the method is Post Office Protocol, revision 3 or POP3.
In order to use the POP3 an email client such as Outlook Express need to know POP3 server address, user id (login) and password. The POP3 server understands a very simple set of text commands. Here are the most common commands:
- USER - enter your user ID
- PASS - enter your password
- QUIT - quit the POP3 server
- LIST - retrieve a list the messages (as a list of ids)
- RETR - retrieve a message specified by an id
- DELE - delete a message specified by id
For a complete POP3 specification please refer to the RFC 1225.
The email client connects to the POP3 server and issues a series of commands to bring copies of email messages to the local computer. Generally, it will then delete the messages from the server, unless it has been told not to ("the leave email on a server" or similar option).
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