Post office protocol

In computing, the Post Office Protocol (POP3, Post Office Protocol or "Post Office Protocol" is used 34;) on local mail clients to get email messages stored on a remote server, called the POP Server. It is an application layer protocol in the OSI Model.

The versions of the POP protocol, informally known as POP1 (RFC 918) and POP2, (RFC 937) have been made obsolete by the latest versions of POP3. In general when referring to the term POP, it refers to POP3 within the context of email protocols.

Features

POP3 is designed for receiving mail, which in some cases is not designed for sending; this allows users with intermittent or very slow connections (such as modem connections) to download their email while online and check it later even when offline. It should be mentioned that although some email clients include the option to leave messages on the server, the general operation is: a client that uses POP3 connects, gets all the messages, stores them on the client's computer, user as new messages, deletes them from the server and finally disconnects. In contrast, the IMAP protocol allows both connected and offline modes of operation.

Email clients using IMAP typically leave messages on the server until the user directly deletes them. This and other factors cause IMAP operation to allow multiple clients to access the same mailbox. Most email clients support POP3 or IMAP; however, only a few Internet providers offer IMAP as an added value to their services.

Clients that use the leave messages on the server option typically use the UIDL command (Unique IDentification Listing). Most POP3 commands identify messages depending on their ordinal number from the mail server. This creates problems when a client tries to leave messages on the server, since the numbered messages change from one connection to the server to another. For example, a mailbox contained 5 messages in the last connection, then another client deletes message number 3, if another connection is started again, the number that message 4 has will become 3, and message 5 will become be number 4 and the direction of these two messages will change. The UIDL provides a mechanism that avoids numbering problems. The server assigns a unique and permanent string to the message. When a POP3 compliant mail client connects to the server it uses the UIDL command to obtain the message identifier mapping. In this way the client can use this mapping to determine which messages to download and which to save at download time.

Like other older internet protocols, POP3 used a signing mechanism without encryption. The transmission of POP3 passwords in plain text still occurs. POP3 currently has various authentication methods that offer a diverse range of levels of protection against illegal access to users' mailboxes. One of these is APOP, which uses MD5 functions to prevent password attacks. Mozilla, Eudora, Novell Evolution as well as Mozilla Thunderbird implement APP functions.

Orders

To establish a connection to a POP server, the mail client opens a TCP connection on port 110 of the server. When the connection is established, the POP server sends the POP client, and then the two machines send each other other commands and responses that are specified in the protocol. As part of this communication, the POP client is asked to authenticate itself (Authentication State), where the user's username and password are sent to the POP server. If the authentication is successful, the POP client enters the Transaction State, in this state LIST, RETR, and DELE commands can be used to display, download, and delete messages from the server, respectively. Messages set for deletion are not actually removed from the server until the POP client issues the QUIT command to terminate the session. At this point, the POP server enters the Update State, where flagged messages are removed and all remaining session resources are cleaned up.

It is possible to manually connect to the POP3 server by Telnetting to port 110. It is very useful when they send a message with a very long file that they do not want to receive.

• USER ≤3000 User ID (It is only done once).
• PASS word PDF Send the server key.
• STAT It gives the number of messages not deleted in the mailbox and its total length.
• LIST It shows all the un erased messages with their length.
• RETR Δnúmeroγ Request the sending of the message specifying the number (not deleted from the mailbox).
• TOP Δnúmeroγ lines It shows the header and the number of required lines of the message specifying the number.
• DELE ≥númeroѕ Delete the message specifying the number.
• RSET Recovers deleted messages (in the current connection).
• UIDL Δnúmero It returns an identifier of the persistent message through the sessions. If it is not specified θnúmero HCFC is returned a list with the messages numbers and their identifier of the un erased messages.
• QUIT Exit.

Advantages

The advantage with other protocols is that between server-client you don't have to send so many orders for the communication between them. The POP protocol also works fine if you don't use a constant connection to the Internet or the network that contains the mail server.