POP - Post Office Protocol

Post Office Protocol (POP) is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection. The Post Office Protocol (POP) allows you to fetch email that is waiting in a mail server mailbox. POP defines a number of operations for how to access and store email on your server.
It works in conjunction with the SMTP (Simple Mail Transfer Protocol), which provides the message transport services required to move mail from one system to another.

Purpose of POP

If somebody sends you an email it usually cannot be delivered directly to your computer. The message is stored in a place where you can pick it up easily. Your ISP (Internet Service Provider) does this job. It receives the message for you and keeps it until you download it. Now, POP, the Post Office Protocol is what allows you to retrieve mail from your ISP. This is also about all the Post Office Protocol is good for.

What POP allows you to do?

Things that can be done via the POP include:
- Retrieve mail from an ISP and delete it on the server.
- Retrieve mail from an ISP but not delete it on the server.
- Ask whether new mail has arrived but not retrieve it.
- Peek at a few lines of a message to see whether it is worth retrieving.

POP3 is an open Internet standard. The common POP3 commands and responses are :
- getwelcome(): Gets the greeting from the server.
- user(username): Login with a username. If valid username, server will respond with request for password.
- pass_ (password): Send password. If valid, server response will be two numbers, message count and mailbox size.
- stat(): Get the mailbox status. Response is two numbers, message count and mailbox size.
- list([message]): Get list of messages. An option "message" gets information on a specific message.
- retr(message): Get message number "message".
- dele(message): Delete message number "message".
- rset(): Remove all deleted message markings.
- noop(): No operation. Do nothing. Really. Needed in unusual programming situations.
- quit(): Quit. Commits all changes, unlocks the mailbox, and ends the server connection.
- top(message, lines): Gets just the first "lines" number of lines of message number "message". Useful on low bandwidth lines to get just the first part of long messages.
- uidl([message]): Gets a unique id list -- a message digest including unique ids. The option gets the unique id for the specific message "message".