In computing, local e-mail clients use the Post Office Protocol version 3 (POP3), an application-layer Internet standard protocol, to retrieve e-mail from a remote server over a TCP/IP connection. Computing is usually defined like the activity of using and developing Computer technology Computer hardware and software. An e-mail client, aka Mail User Agent (MUA aka e-mail reader is a frontend Computer program used to manage E-mail. The Application Layer is the seventh level of the seven-layer OSI model, and the top layer of the TCP/IP model In Computer network Engineering, an Internet Standard (STD is a Specification, put forward by the Internet Engineering Task Force (IETF for In computing, a protocol is a convention or standard that controls or enables the connection Communication, and Data transfer between two computing Electronic mail, often abbreviated to e-mail, email, or originally eMail, is a Store-and-forward method of writing sending receiving A mail transfer agent (MTA (also called a mail transport agent, message transfer agent, or smtpd (short for SMTP daemon) is a The Internet Protocol Suite (commonly TCP/IP) is the set of Communications protocols used for the Internet and other similar networks POP3 and IMAP4 (Internet Message Access Protocol) are the two most prevalent Internet standard protocols for e-mail retrieval. The Internet Message Access Protocol or IMAP is one of the two most prevalent Internet standard protocols for E-mail retrieval the other being POP3 Virtually all modern e-mail clients and servers support both.
POP3 has made earlier versions of the protocol, informally called POP1 and POP2, obsolete. In contemporary usage, the less precise term POP almost always means POP3 in the context of e-mail protocols.
The design of POP3 and its procedures supports end-users with intermittent connections (such as dial-up connections), allowing these users to retrieve e-mail when connected and then to view and manipulate the retrieved messages without needing to stay connected. Dial-up Internet Access is a form of Internet access via Telephone lines The user's computer or Router uses an attached Modem connected to a 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. The Internet Message Access Protocol or IMAP is one of the two most prevalent Internet standard protocols for E-mail retrieval the other being POP3 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. An Internet service provider ( ISP, also called Internet access provider or IAP) is a company which primarily offers their customers access to the Internet The fundamental difference between POP3 and IMAP4 is that POP3 offers access to a mail drop; the mail exists on the server until it is collected by the client. Even if the client leaves some or all messages on the server, the client's message store is considered authoritative. In contrast, IMAP4 offers access to the mail store; the client may store local copies of the messages, but these are considered to be a temporary cache; the server's store is authoritative.
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. A mail transfer agent (MTA (also called a mail transport agent, message transfer agent, or smtpd (short for SMTP daemon) is a 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. For other uses see Character. In Computer and machine-based Telecommunications terminology a character is a unit of 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.
Whether using POP3 or IMAP to retrieve messages, e-mail clients typically use the SMTP_Submit profile of the Simple Mail Transfer Protocol (SMTP) to send messages. Simple Mail Transfer Protocol ( SMTP) is a De facto standard for electronic mail (e-mail transmissions across the Internet. E-mail clients are commonly categorized as either POP or IMAP clients, but in both cases the clients also use SMTP. There are extensions to POP3 that allow some clients to transmit outbound mail via POP3 - these are known as "XTND XMIT" extensions. The Qualcomm qpopper and CommuniGate Pro servers and Eudora clients are examples of systems that optionally utilize the XTND XMIT methods of authenticated client-to-server e-mail transmission.
MIME serves as the standard for attachments and non-ASCII text in e-mail. Multipurpose Internet Mail Extensions ( MIME) is an Internet standard that extends the format of e-mail to support text in Character American Standard Code for Information Interchange ( ASCII) Although neither POP3 nor SMTP require MIME-formatted e-mail, essentially all Internet e-mail comes MIME-formatted, so POP clients must also understand and use MIME. IMAP, by design, assumes MIME-formatted e-mail.
Like many other older Internet protocols, POP3 originally supported only an unencrypted login mechanism. Although plain text transmission of passwords in POP3 still commonly occurs, POP3 currently supports several authentication methods to provide varying levels of protection against illegitimate access to a user's e-mail. In Computing, plain text is a term used for an ordinary "unformatted" sequential file readable as textual material without much processing Authentication (from Greek αυθεντικός real or genuine from authentes author is the act of establishing or confirming something (or someone as One such method, APOP, uses the MD5 hash function in an attempt to avoid replay attacks and disclosure of the shared secret. In Cryptography, MD5 ( Message-Digest algorithm 5) is a widely used partially insecure Cryptographic hash function with a 128- Bit hash value A cryptographic Hash function is a transformation that takes an input (or 'message' and returns a fixed-size string which is called the hash value (sometimes A replay attack is a form of network attack in which a valid data transmission is maliciously or fraudulently repeated or delayed In Cryptography, a shared secret is a piece of data only known to the parties involved in a secure communication Clients implementing APOP include Mozilla Thunderbird, Opera, Eudora, KMail, Novell Evolution, Windows Live Mail, and PowerMail. Mozilla Thunderbird is a free, Open source, Cross-platform e-mail and News client developed by the Mozilla Foundation. Opera is a Web browser and Internet suite developed by the Opera Software company Eudora is an E-mail client used on the Apple Macintosh and Microsoft Windows Operating systems It also supports several palmtop computing KMail is the E-mail client of the KDE Desktop environment. It supports folders filtering viewing HTML mail and international character Evolution or Novell Evolution (formerly Ximian Evolution, prior to Novell 's 2003 acquisition of Ximian) is the official Personal information Windows Live Mail (formerly named Windows Live Mail Desktop, code-named Elroy) is an E-mail client from Microsoft 's Windows Live POP3 clients can also support SASL authentication methods via the AUTH extension. Simple Authentication and Security Layer ( SASL) is a Framework for Authentication and Data security in Internet protocols It decouples MIT Project Athena also produced a Kerberized version. Project Athena was a joint project of MIT, Digital Equipment Corporation, and IBM. Kerberos is a Computer network Authentication protocol, which allows individuals communicating over a non-secure network to prove their identity to one
POP3 works over a TCP/IP connection using TCP on network port 110. In Computer networking, a port is an application-specific or process-specific software construct serving as a communications endpoint used by Transport Layer protocols E-mail clients can encrypt POP3 traffic using TLS or SSL. Transport Layer Security ( TLS) and its predecessor Secure Sockets Layer ( SSL) are Cryptographic protocols that provide secure Transport Layer Security ( TLS) and its predecessor Secure Sockets Layer ( SSL) are Cryptographic protocols that provide secure A TLS or SSL connection is negotiated using the STLS command. Transport Layer Security ( TLS) and its predecessor Secure Sockets Layer ( SSL) are Cryptographic protocols that provide secure Transport Layer Security ( TLS) and its predecessor Secure Sockets Layer ( SSL) are Cryptographic protocols that provide secure Some clients and servers, like Google Gmail, instead use the deprecated alternate-port method, which uses TCP port 995.
RFC 1939 APOP support indicated by <1896. 697170952@dbc. mtview. ca. us> here:
S: <wait for connection on TCP port 110>C: <open connection>S: +OK POP3 server ready <1896. 697170952@dbc. mtview. ca. us>C: APOP mrose c4c9334bac560ecc979e58001b3e22fbS: +OK mrose's maildrop has 2 messages (320 octets)C: STATS: +OK 2 320C: LISTS: +OK 2 messages (320 octets)S: 1 120S: 2 200S: . C: RETR 1S: +OK 120 octetsS: <the POP3 server sends message 1>S: . C: DELE 1S: +OK message 1 deletedC: RETR 2S: +OK 200 octetsS: <the POP3 server sends message 2>S: . C: DELE 2S: +OK message 2 deletedC: QUITS: +OK dewey POP3 server signing off (maildrop empty)C: <close connection>S: <wait for next connection>
POP3 servers without the optional APOP command expect you to log in with the USER and PASS commands:
C: USER mroseS: +OK User acceptedC: PASS mrosepassS: +OK Pass accepted
While not yet an official standardized mail protocol, a proposal has been outlined for a POP4 specification, complete with a working server implementation.
The proposed POP4 extension adds basic folder management, multipart message support, as well as message flag management, allowing for a light protocol which supports some popular IMAP features which POP3 currently lacks.
No progress has been observed in the POP4 specification since 2003.
Demon Internet introduced extensions to POP3 that allow multiple accounts per domain, and has become known as Standard Dial-up POP3 Service (SDPS). Demon Internet Demon Internet is a British Internet Service Provider. 
To access each account, the username includes the hostname, as john@hostname or john+hostname.