Information about Post Office Protocol
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. Many subscribers to individual Internet service provider e-mail accounts access their e-mail with client software that uses POP3.
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. 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. 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. 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, using a 32-bit UID (Unique IDentifier) that is required to be strictly ascending. The advantage of the numeric UID is with large mailboxes; a client can request just the UIDs greater than its previously stored "highest UID". In POP, the client must fetch the entire UIDL map.
Whether using POP3 or IMAP to retrieve messages, e-mail clients typically use the SMTP_Submit profile of the SMTP protocol to send messages. 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. 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. One such method, APOP, uses the MD5 hash function in an attempt to avoid replay attacks and disclosure of the shared secret. Clients implementing APOP include Mozilla Thunderbird, Opera, Eudora, KMail and Novell Evolution. POP3 clients can also support SASL authentication methods via the AUTH extension. MIT Project Athena also produced a Kerberized version.
POP3 works over a TCP/IP connection using TCP on network port 110. E-mail clients can encrypt POP3 traffic using TLS or SSL. A TLS or SSL connection is negotiated using the STLS command. Some clients and servers, like Google Gmail, instead use the deprecated alternate-port method, which uses TCP port 995.
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 c4c9334bac560ecc979e58001b3e22fb S: +OK mrose's maildrop has 2 messages (320 octets) C: STAT S: +OK 2 320 C: LIST S: +OK 2 messages (320 octets) S: 1 120 S: 2 200 S: . C: RETR 1 S: +OK 120 octets S: <the POP3 server sends message 1> S: . C: DELE 1 S: +OK message 1 deleted C: RETR 2 S: +OK 200 octets S: <the POP3 server sends message 2> S: . C: DELE 2 S: +OK message 2 deleted C: QUIT S: +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 mrose S: +OK User accepted C: PASS mrosepass S: +OK Pass accepted
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.
To access each account, the username includes the hostname, as john@hostname or john+hostname.
Overview
POP3 has made earlier versions of the protocol obsolete, POP (informally called POP1 and POP2). 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. 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. 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. 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, using a 32-bit UID (Unique IDentifier) that is required to be strictly ascending. The advantage of the numeric UID is with large mailboxes; a client can request just the UIDs greater than its previously stored "highest UID". In POP, the client must fetch the entire UIDL map.
Whether using POP3 or IMAP to retrieve messages, e-mail clients typically use the SMTP_Submit profile of the SMTP protocol to send messages. 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. 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. One such method, APOP, uses the MD5 hash function in an attempt to avoid replay attacks and disclosure of the shared secret. Clients implementing APOP include Mozilla Thunderbird, Opera, Eudora, KMail and Novell Evolution. POP3 clients can also support SASL authentication methods via the AUTH extension. MIT Project Athena also produced a Kerberized version.
POP3 works over a TCP/IP connection using TCP on network port 110. E-mail clients can encrypt POP3 traffic using TLS or SSL. A TLS or SSL connection is negotiated using the STLS command. Some clients and servers, like Google Gmail, instead use the deprecated alternate-port method, which uses TCP port 995.
Dialog example
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 c4c9334bac560ecc979e58001b3e22fb S: +OK mrose's maildrop has 2 messages (320 octets) C: STAT S: +OK 2 320 C: LIST S: +OK 2 messages (320 octets) S: 1 120 S: 2 200 S: . C: RETR 1 S: +OK 120 octets S: <the POP3 server sends message 1> S: . C: DELE 1 S: +OK message 1 deleted C: RETR 2 S: +OK 200 octets S: <the POP3 server sends message 2> S: . C: DELE 2 S: +OK message 2 deleted C: QUIT S: +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 mrose S: +OK User accepted C: PASS mrosepass S: +OK Pass accepted
POP4
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.
SDPS
Demon Internet introduced extensions to POP3 that allow multiple accounts per domain, and has become known as Standard Dial-up POP3 Service (SDPS).[1]To access each account, the username includes the hostname, as john@hostname or john+hostname.
See also
- Internet Message Access Protocol (IMAP)
- Simple Mail Transfer Protocol (SMTP)
- E-mail client
- webmail
- POP3 clients: getmail, fetchmail
- email encryption
External links
POP3 RFCs
- RFC 1939 - "Post Office Protocol - Version 3"
- RFC 2195 - "IMAP/POP AUTHorize Extension for Simple Challenge/Response"
- RFC 2449 - "POP3 Extension Mechanism"
- RFC 1734 - "POP3 AUTHentication command"
- RFC 2222 - "Simple Authentication and Security Layer (SASL)"
- RFC 3206 - "The SYS and AUTH POP Response Codes"
- RFC 2595 - "Using TLS with IMAP, POP3 and ACAP"
- RFC 937 - "POST OFFICE PROTOCOL - VERSION 2"
- RFC 918 - "POST OFFICE PROTOCOL"
Server implementations
- Qpopper
- popa3d
- Dovecot
- Teapop
- Apache James
- Zimbra
- Citadel/UX
- UW IMAP
- Cyrus IMAP server
- qmail-qpop3d
- Eudora Internet Mail Server
- IUPOP3
computing is synonymous with counting and calculating. Originally, people that performed these functions were known as computers. Today it refers to a science and technology that deals with the computation and the manipulation of symbols.
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An e-mail client is a frontend computer program used to manage e-mail. Large all-in-one e-mail clients such as the open source Mozilla Thunderbird and Microsoft Outlook today combine the operations of an MSA, MDA, MRA and MUA in one application.
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application layer is the seventh level of the seven-layer OSI model. It interfaces directly to and performs common application services for the application processes; it also issues requests to the presentation layer.
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An Internet standard is a specification for an innovative internetworking technology or methodology, which the Internet Engineering Task Force (IETF) ratified as an open standard after the innovation underwent peer review.
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protocol is a convention or standard that controls or enables the connection, communication, and data transfer between two computing endpoints. In its simplest form, a protocol can be defined as the rules governing the syntax, semantics, and synchronization of communication.
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E-mail (short for electronic mail; often also abbreviated as e-mail, email or simply mail) is a store and forward method of composing, sending, storing, and receiving messages over electronic communication systems.
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A mail transfer agent or MTA (also called a mail transport agent, message transfer agent, mail server, SMTPD (short for SMTP daemon), or a mail exchanger
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The Internet protocol suite is the set of communications protocols that implement the protocol stack on which the Internet and most commercial networks run. It has also been referred to as the TCP/IP protocol suite, which is named after two of the most important protocols in it:
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Internet service provider (abbr. ISP, also called Internet access provider or IAP) is a business or organization that provides consumers or businesses access to the Internet and related services. In the past, most ISPs were run by the phone companies.
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hot Dial-up access is a form of Internet access via telephone line. The client uses a modem connected to a computer and a telephone line to dial into an Internet service provider's (ISP) node to establish a modem-to-modem link, which is then routed to the Internet.
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The Internet Message Access Protocol (commonly known as IMAP or IMAP4, and previously called Internet Mail Access Protocol, Interactive Mail Access Protocol (RFC 1064), and Interim Mail Access Protocol[1]
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Internet service provider (abbr. ISP, also called Internet access provider or IAP) is a business or organization that provides consumers or businesses access to the Internet and related services. In the past, most ISPs were run by the phone companies.
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A mail transfer agent or MTA (also called a mail transport agent, message transfer agent, mail server, SMTPD (short for SMTP daemon), or a mail exchanger
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character is a unit of information that roughly corresponds to a grapheme, grapheme-like unit, or symbol, such as in an alphabet or syllabary in the written form of a natural language.
An example of a character is a letter, numeral, or punctuation mark.
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An example of a character is a letter, numeral, or punctuation mark.
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Simple Mail Transfer Protocol (SMTP) is the de facto standard for e-mail transmissions across the Internet. Formally SMTP is defined in RFC 821 (STD 10) as amended by RFC 1123 (STD 3) chapter 5. The protocol used today is also known as ESMTP and defined in RFC 2821.
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Mime or pantomime is a theatrical medium or performance art, involving the acting out of a story by a mime artist through body motions, without use of speech.
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History
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American Standard Code for Information Interchange (ASCII), generally pronounced ask-ee IPA: /ˈæski/ ( [1] ), is a character encoding based on the English alphabet.
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encryption is the process of transforming information (referred to as plaintext) to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key.
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plain text is textual material in a computer file which is unformatted and without very much processing readable by simple computer tools such as line printing text commands, in Windows'es DOS window
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Authentication (from Greek αυθεντικός; real or genuine, from authentes; author) is the act of establishing or confirming something (or someone) as authentic, that is, that claims made by or about the thing are true.
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MD5
General
Ronald Rivest
April 1992
MD, MD2, MD3, MD4, MD5
Detail
128 bits
4
In cryptography, MD5 (Message-Digest algorithm 5) is a widely used cryptographic hash function with a 128-bit hash value.
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General
Ronald Rivest
April 1992
MD, MD2, MD3, MD4, MD5
Detail
128 bits
4
In cryptography, MD5 (Message-Digest algorithm 5) is a widely used cryptographic hash function with a 128-bit hash value.
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In cryptography, a cryptographic hash function is a transformation that takes an input and returns a fixed-size string, which is called the hash value. Hash functions with this property are used for a variety of computational purposes, including cryptography.
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A replay attack is a form of network attack in which a valid data transmission is maliciously or fraudulently repeated or delayed. This is carried out either by the originator or by an adversary who intercepts the data and retransmits it, possibly as part of a masquerade attack by
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shared secret is a piece of data only known to the parties involved in a secure communication. The shared secret can be a password, a passphrase, a big number or an array of randomly chosen bytes.
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Maintainer: Mozilla Corporation
OS: Cross-platform
Available language(s): Multilingual
Use: E-mail client and news client
License: MPL/GPL/LGPL tri-license
Website: www.mozilla.
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OS: Cross-platform
Available language(s): Multilingual
Use: E-mail client and news client
License: MPL/GPL/LGPL tri-license
Website: www.mozilla.
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Maintainer: Opera Software ASA
OS: Cross-platform
Use: Internet suite
License: Proprietary
Website: [1] Opera is a cross-platform web browser and Internet suite developed by the Opera Software corporation.
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OS: Cross-platform
Use: Internet suite
License: Proprietary
Website: [1] Opera is a cross-platform web browser and Internet suite developed by the Opera Software corporation.
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Eudora is an e-mail client used on the Apple Macintosh and Microsoft Windows operating systems. It also supports several palmtop computing platforms, including Newton and the Palm OS. The software was named after Eudora Welty because of her short story "Why I Live at the P.O.
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KMail is the e-mail client of the KDE desktop environment.
It supports folders, filtering, viewing HTML mail, and international character sets. It can handle IMAP, dIMAP [1] , POP3, and local mailboxes for incoming mail. It can send mail via SMTP or sendmail.
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It supports folders, filtering, viewing HTML mail, and international character sets. It can handle IMAP, dIMAP [1] , POP3, and local mailboxes for incoming mail. It can send mail via SMTP or sendmail.
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Evolution or Novell Evolution (formerly Ximian Evolution, prior to Novell's 2003 acquisition of Ximian) is the official personal information manager and workgroup information management tool for GNOME.
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Simple Authentication and Security Layer (SASL) is a framework for authentication and data security in Internet protocols. It decouples authentication mechanisms from application protocols, in theory allowing any authentication mechanism supported by SASL to be used in any
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