09Mar Post Office Protocol ver. 3 (POP3)

POP3 protocol is basically used for sending and receiving mails from and to your desktop. In computing, the Post Office Protocol version 3 (POP3) 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. POP3 and IMAP4 (Internet Message Access Protocol) are the two most prevalent Internet standard protocols for e-mail retrieval. Virtually all modern e-mail clients and servers support both.

POP3 protocol is basically used for sending and receiving mails from and to your desktop. In computing, the Post Office Protocol version 3 (POP3) 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. POP3 and IMAP4 (Internet Message Access Protocol) are the two most prevalent Internet standard protocols for e-mail retrieval. Virtually all modern e-mail clients and servers support both.

The design of POP3 and its procedures supports end-users with intermittent connections, allowing these users to retrieve e-mail when connected and then to view and manipulate the retrieved messages without needing to stay connected. Although most clients have an option to leave mail on server, e-mail clients using POP3 generally connect, retrieve all messages, store them on the user’s PC as new messages, delete them from the server, and then disconnect. In contrast, the newer, more capable Internet Message Access Protocol (IMAP) supports both connected (online) and disconnected (offline) modes of operation. E-mail clients using IMAP generally leave messages on the server until the user explicitly deletes them. This and other aspects of IMAP operation allow multiple clients to access the same mailbox. Most e-mail clients support either POP3 or IMAP to retrieve messages; however, fewer Internet Service Providers (ISPs) support IMAP.

Clients with a leave mail on server option generally use the POP3 UIDL (Unique IDentification Listing) command. Most POP3 commands identify specific messages by their ordinal number on the mail server. This creates a problem for a client intending to leave messages on the server, since these message numbers may change from one connection to the server to another. For example if a mailbox contains five messages at last connect, and a different client then deletes message #3, the next connecting user will find the last two messages’ numbers decremented by one. UIDL provides a mechanism to avoid these numbering issues. The server assigns a string of characters as a permanent and unique ID for the message. When a POP3-compatible e-mail client connects to the server, it can use the UIDL command to get the current mapping from these message IDs to the ordinal message numbers. The client can then use this mapping to determine which messages it has yet to download, which saves time when downloading. IMAP has a similar mechanism, a 32-bit unique identifier (UID) that must be assigned to messages in ascending (although not necessarily consecutive) order as they are received. Because IMAP UIDs are assigned in this manner, to retrieve new messages an IMAP client need only request the UIDs greater than the highest UID among all previously-retrieved messages, whereas a POP client must fetch the entire UIDL map. For large mailboxes, this difference can be significant.