About POP3 SMTP email protocol, mail readers

POP3 session is created when
a remote user connects to a mailbox on the POP3 mail server
and ends when the remote user disconnects from the POP3 mail server.

About POP3 Protocol

Post Office Protocol Version 3 (POP3) is a standard internet protocol, which was developed as the standard client-server protocol for retrieving Internet mail by using an email client ie: Outlook Express, Netscape Communicator, Eudora, etc. to communicate with the mail server (Protocol, a method of sending and receiving data).

POP3 - Post Office Protocol Version 3 rfc 1939 is used for delivering messages to (client) mail readers (fetch mails from a remote mail server and store it locally).

The client sends a login (UserID) and a password to authenticate the session. If it is accepted--the client sends commands to retrieve messages. The server transfers the messages and deletes them from the permanent mailbox.

By default, your POP3 clients login to the ISP POP3 server by sending the password for the mail account and a POP3 session is created when you (a remote user) connects to a mailbox on your ISP POP3 mail server. The session ends when you (the remote user) disconnects from your ISP POP3 mail server.

POP3 mail server listen for mail requests and forward mail to the requesting party. The default port on which the POP3 server listens for these requests is 110. Client side mail readers can issue a predefined set of commands to the mail server and access the mail.

By default, POP3 client's login to the POP3 server by sending the password (as text) for the mail account. This may be undesirable in high security-type situations. With POP3's optional APOP authentication, the password itself is never sent to the remote mail server. Instead, a digest is created using the RSA MD5 algorithm.

This digest is based on the password and a unique value supplied by the POP3 mail server during the connection. This digest is sent to the mail server in lieu of the password. Because the unique value supplied by the mail server changes with each connection, A POP authentication ensures that a different digest will be sent each time you log onto the sever and reduces the ability for a network sniffing program to capture password.

APOP is a POP3 setting that encrypts your username and password, an authentication mechanism designed to protect your POP3 account password when checking mail.

APOP authentication does not require your account password to be sent as plain text to the POP3 mail server, however, not all Internet providers support it.

A POP3 session is created when a remote user connects to a mailbox on the POP3 mail server. The session ends when the remote user disconnects from the POP3 mail server. While a session is open, no other user can retrieve mail from the POP3 mailbox, however mail can still be sent to that account. This is by design of the POP3 protocol.

While in a session, regardless of new mail which may be arriving in the mailbox, all things remain equal. At the time the session is initiated, mailbox state is saved and the entire session operates off of that cached state. Once a session is terminated, values obtained from that session may no longer be valid.

Therefore, if you initiate a session with a POP3 mail server and find that there are five messages available. While in the session, operations on these messages require a message number, which is the position of the message in the list of five.

Once you terminate your session, you can not assume that five messages are still available. If you initiate another session, you can not assume that the message number three from the first session is message number three in the new session.

Because of this, certain message operations are only valid while in the same session which downloaded the message. Once a session is terminated, if you wish to perform server operations on a specific message, you will need to establish a new session, locate the message by comparing its properties (subject, date, message id, etc...) to ones you saved from the previous session, and then act upon the message before terminating the new session.

See Post Office Protocol (POP3 RFC 1939) Version 3 - The Standard internet protocol used for retrieving Internet email.

Other protocols involved in sending and receiving of e-mails.

MTA - Message Transfer Agent the exchange of mail using TCP/IP is performed by MTA that defines how email messages are transmitted between two users.

Email Daemon - An email software program residing in a mail server that manages incoming and outgoing mails.

<<back to>>
eMail Bolts&Nuts