Microsoft Exchange Server

Microsoft Exchange Server is a mail server and calendaring server developed by Microsoft. It runs exclusively on Windows Server operating systems.

Exchange Server was initially Microsoft’s internal mail server. The first version of Exchange Server to be published outside Microsoft was Exchange Server 4.0. Exchange initially used the X.400 directory service but switched to Active Directory later.

X.400 is a suite of ITU-T Recommendations that define standards for Data Communication Networks for Message Handling Systems (MHS) — more commonly known as email

The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU); it coordinates standards for telecommunications.The standardization efforts of ITU commenced in 1865 with the formation of the International Telegraph Union (ITU). ITU became a Specialized agency of the United Nations in 1947. The International Telegraph and Telephone Consultative Committee (CCITT, from French: Comité Consultatif International Téléphonique et Télégraphique) was created in 1956, and was renamed ITU-T in 1993. ITU-T has a permanent secretariat, the Telecommunication Standardization Bureau (TSB), based at the ITU headquarters in Geneva, Switzerland. The current Director of the Bureau is Chaesub Lee, whose 4-year term commenced on 1 January 2015, who replaced Malcolm Johnson of the United Kingdom, who was director from 1 January 2007 to 2014.

Primary function of ITU-T:

The ITU-T mission is to ensure the efficient and timely production of standards covering all fields of telecommunications on a worldwide basis, as well as defining tariff and accounting principles for international telecommunication services.

The international standards that are produced by the ITU-T are referred to as “Recommendations” (with the word ordinarily capitalized to distinguish its meaning from the ordinary sense of the word “recommendation”), as they become mandatory only when adopted as part of a national law.

Since the ITU-T is part of the ITU, which is a United Nations specialized agency, its standards carry more formal international weight than those of most other standards development organizations that publish technical specifications of a similar form)

At one time, the designers of X.400 were expecting it to be the predominant form of email, but this role has been taken by the SMTP-based Internet e-mail. Despite this, it has been widely used within organizations and was a core part of Microsoft Exchange Server until 2006; variants continue to be important in military and aviation contexts ]

 Versions 4.0 and 5.0 came bundled with an email client called Microsoft Exchange Client. It was discontinued in favor of Microsoft Outlook.

Exchange Server uses a proprietary protocol called MAPI. Over time, however, it added support for POP3IMAPSMTP, and EAS

Post Office Protocol

POP – In computing, the Post Office Protocol (POP) is an application-layerInternet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection.[1] POP has been developed through several versions, with version 3 (POP3) being the last standard in common use before largely being made obsolete by the more advanced IMAP as well as webmail

Overview of POP

POP supports download-and-delete requirements for access to remote mailboxes (termed maildrop in the POP RFC‘s). Although most POP clients have an option to leave mail on server after download, e-mail clients using POP generally connect, retrieve all messages, store them on the user’s PC as new messages, delete them from the server, and then disconnect. Other protocols, notably IMAP, (Internet Message Access Protocol) provide more complete and complex remote access to typical mailbox operations. In the late 1990s and early 2000s, fewer Internet Service Providers (ISPs) supported IMAP due to the storage space that was required on the ISP’s hardware. Contemporary e-mail clients supported POP, then over time popular mail client software added IMAP support.

A POP3 server listens on well-known port 110. Encrypted communication for POP3 is either requested after protocol initiation, using the STLS command, if supported, or by POP3S, which connects to the server using Transport Layer Security (TLS) or Secure Sockets Layer (SSL) on well-known TCP port 995.

Available messages to the client are fixed when a POP session opens the maildrop, and are identified by message-number local to that session or, optionally, by a unique identifier assigned to the message by the POP server. This unique identifier is permanent and unique to the maildrop and allows a client to access the same message in different POP sessions. Mail is retrieved and marked for deletion by message-number. When the client exits the session, the mail marked for deletion is removed from the maildrop.

Internet Message Access Protocol 

IMAP – In computing, the Internet Message Access Protocol (IMAP) is an Internet standard protocol used by e-mail clients to retrieve e-mail messages from a mail server over a TCP/IP connection. IMAP is defined by RFC 3501.IMAP was designed with the goal of permitting complete management of an email box by multiple email clients, therefore clients generally leave messages on the server until the user explicitly deletes them. An IMAP server typically listens on port number 143. IMAP over SSL (IMAPS) is assigned the port number 993.Virtually all modern e-mail clients and servers support IMAP. IMAP and the earlier POP3 (Post Office Protocol) are the two most prevalent standard protocols for email retrieval,[2] with many webmail service providers such as GmailOutlook.com and Yahoo! Mail also providing support for either IMAP or POP3.

E-mail protocols of IMAP

The Internet Message Access Protocol is an Application Layer Internet protocol that allows an e-mail client to access e-mail on a remote mail server. The current version, IMAP version 4 revision 1 (IMAP4rev1), is defined by RFC 3501. An IMAP server typically listens on well-known port 143. IMAP over SSL (IMAPS) is assigned well-known port number 993.

IMAP supports both on-line and off-line modes of operation. E-mail clients using IMAP generally leave messages on the server until the user explicitly deletes them. This and other characteristics of IMAP operation allow multiple clients to manage the same mailbox. Most e-mail clients support IMAP in addition to Post Office Protocol (POP) to retrieve messages; however, fewer e-mail services support IMAP.[3] IMAP offers access to the mail storage. Clients may store local copies of the messages, but these are considered to be a temporary cache.

Incoming e-mail messages are sent to an e-mail server that stores messages in the recipient’s e-mail box. The user retrieves the messages with an e-mail client that uses one of a number of e-mail retrieval protocols. Some clients and servers preferentially use vendor-specific, proprietary protocols, but most support SMTP for sending e-mail and POP and IMAP for retrieving e-mail, allowing interoperability with other servers and clients. For example, Microsoft‘s Outlook client uses MAPI, a Microsoft proprietary protocol, to communicate with a Microsoft Exchange ServerIBM‘s Notes client works in a similar fashion when communicating with a Domino server. All of these products also support POP, IMAP, and outgoing SMTP. Support for the Internet standard protocols[citation needed] allows many e-mail clients such as Pegasus Mail or Mozilla Thunderbird to access these servers, and allows the clients to be used with other servers.

Simple Mail Transfer Protocol 

SMTP – Simple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail (email) transmission. First defined by RFC 821 in 1982, it was last updated in 2008 with Extended SMTP additions by RFC 5321, which is the protocol in widespread use today.Although electronic mail servers and other mail transfer agents use SMTP to send and receive mail messages, user-level client mail applications typically use SMTP only for sending messages to a mail server for relaying. For retrieving messages, client applications usually use either IMAP or POP3.SMTP communication between mail servers uses port 25. Mail clients on the other hand, often submit the outgoing emails to a mail server on port 587. Despite being deprecated, mail providers sometimes still permit the use of nonstandard port 465 for this purpose.SMTP connections secured by SSL, known as SMTPS, can be made using STARTTLS.[1]Although proprietary systems (such as Microsoft Exchange and IBM Notes) and webmail systems (such as Outlook.comGmail and Yahoo! Mail) use their own non-standard protocols to access mail box accounts on their own mail servers, all use SMTP when sending or receiving email from outside their own systems.

Mail processing model of SMTP

Email is submitted by a mail client (mail user agent, MUA [also known as Email Client e.g Outlook 2010]) to a mail server (mail submission agent, MSA [A message submission agent (MSA) or mail submission agent is a computer program or software agent that receives electronic mail messages from a mail user agent (MUA) and cooperates with a mail transfer agent (MTA) for delivery of the mail. It uses ESMTP, a variant of the Simple Mail Transfer Protocol (SMTP), as specified in RFC 6409.[1]Many MTAs perform the function of an MSA as well, but there are also programs that are specially designed as MSAs without full MTA functionality. Historically, in Internet mail, both MTA and MSA functions use port number 25, but the official port for MSAs is 587.[1] The MTA accepts incoming mail, while the MSA accepts outgoing mail.]) using SMTP on TCP port 587. Most mailbox providers still allow submission on traditional port 25. 

The MSA delivers the mail to its mail transfer agent (mail transfer agent, MTA). Often, these two agents are instances of the same software launched with different options on the same machine. Local processing can be done either on a single machine, or split among multiple machines; mail agent processes on one machine can share files, but if processing is on multiple machines, they transfer messages between each other using SMTP, where each machine is configured to use the next machine as a smart host. Each process is an MTA (an SMTP server) in its own right. 

The boundary MTA uses the Domain name system (DNS) to look up the mail exchanger record (MX record) for the recipient’s domain (the part of the email address on the right of @). The MX record contains the name of the target host. Based on the target host and other factors, the MTA selects an exchange server: see the article MX record. The MTA connects to the exchange server as an SMTP client.

Message transfer can occur in a single connection between two MTAs, or in a series of hops through intermediary systems. A receiving SMTP server may be the ultimate destination, an intermediate “relay” (that is, it stores and forwards the message) or a “gateway” (that is, it may forward the message using some protocol other than SMTP). Each hop is a formal handoff of responsibility for the message, whereby the receiving server must either deliver the message or properly report the failure to do so.[15]

Once the final hop accepts the incoming message, it hands it to a mail delivery agent (MDA) for local delivery. An MDA saves messages in the relevant mailbox format. As with sending, this reception can be done using one or multiple computers, but in the diagram above the MDA is depicted as one box near the mail exchanger box. An MDA may deliver messages directly to storage, or forward them over a network using SMTP or other protocol such as Local Mail Transfer Protocol (LMTP), a derivative of SMTP designed for this purpose.

Once delivered to the local mail server, the mail is stored for batch retrieval by authenticated mail clients (MUAs). Mail is retrieved by end-user applications, called email clients, using Internet Message Access Protocol (IMAP), a protocol that both facilitates access to mail and manages stored mail, or the Post Office Protocol (POP) which typically uses the traditional mbox mail file format or a proprietary system such as Microsoft Exchange/Outlook or Lotus Notes/DominoWebmail clients may use either method, but the retrieval protocol is often not a formal standard.

SMTP defines message transport, not the message content. Thus, it defines the mail envelope and its parameters, such as the envelope sender, but not the header (except trace information) nor the body of the message itself. STD 10 and RFC 5321 define SMTP (the envelope), while STD 11 and RFC 5322 define the message (header and body), formally referred to as the Internet Message Format.

Exchange ActiveSync 

EAS – Exchange ActiveSync (commonly known as EAS) is a communications protocol designed for the synchronization of email, contacts, calendar, tasks, and notes from a messaging server to a smartphone or other mobile devices. The protocol also provides mobile device management and policy controls. The protocol is based on XML. The mobile device communicates over HTTP or HTTPS. Originally branded as AirSync and only supporting Microsoft Exchange Servers and Pocket PC devices, the protocol has since become a de facto standard for synchronization between groupware and mobile devices.Microsoft licenses the technology. Support for EAS is now implemented in a number of competing collaboration platforms, including GroupWise with the Novell GroupWise Mobility Services software and Lotus Notes with IBM Notes TravelerGoogle previously offered support for the protocol for personal Gmail and free Google Apps accounts, but began removing support from all but paid Google Apps for Work subscriptions in 2013. Beyond on premises installations of Exchange, the various personal and enterprise hosted services from Microsoft also utilize EAS, including Outlook.com and Office 365.In addition to support on Windows Phone, EAS client support is included on AndroidiOSBlackBerry 10 smartphones and the BlackBerry PlayBook tablet computer. The built-in email application for Windows 8 desktop, Mail app, also supports the protocol.

Exchange Server is licensed both in the forms of on-premises software and software as a service. In the on-premises form, customer purchase client access licenses (CALs). In the software as a service form, Microsoft receives a monthly service fee instead (see Microsoft Office 365). 

References:

Microsoft Exchange Server

X.400

ITU-T

Post Office Protocol

Internet Message Access Protocol

Simple Mail Transfer Protocol

Message Submission Agent

Email Client

Exchange ActiveSync

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