SCIENCE DIRECT

Simple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail (e-mail) transmission across Internet Protocol (IP) networks. SMTP was first defined in RFC 821 (STD 10),and last updated by RFC 5321 (2008)which includes the extended SMTP (ESMTP) additions, and is the protocol in widespread use today.

While electronic mail servers and other mail transfer agents use SMTP to send and receive mail messages, user-level client mail applications typically only use SMTP for sending messages to a mail server for relaying. For receiving messages, client applications usually use either the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP) to access their mail box accounts on a mail server.

SMTP is a relatively simple, text-based protocol, in which a mail sender communicates with a mail receiver by issuing simple command strings and supplying necessary data over a reliable ordered data stream channel, typically a Transmission Control Protocol (TCP) connection. An SMTP session consists of a series of commands, initiated by the SMTP client, and responses from the SMTP server through which the session is opened, operating parameters are exchanged, the recipients are specified, and possibly verified, and the message is transmitted, before the session is closed. The originating host is either an end-user's email client also known as mail user agent (MUA), or a relay server's mail transfer agent (MTA).

SMTP was designed as an electronic mail transport and delivery protocol, and as such it is used between SMTP systems that are operational at all times. However, it has capabilities for use as a mail submission protocol for email clients (split user-agent) that do not have the capability to operate as MTA. Such agents are also called message submission agents (MSA), sometimes also referred to as mail submission agents. They are typically end-user applications and send all messages through a smart relay server, often called the outgoing mail server, which is specified in the programs' configuration. A mail transfer agent, incorporated either in the e-mail client directly or in the relay server, typically determines the destination SMTP server by querying the Domain Name System for the mail exchanger (MX record) of each recipient's domain name. Conformant MTAs fall back to a simple address lookup (A record) of the domain name when no mail exchanger is available. In some cases an SMTP client, even a server, may also be configured to use a smart host for delivery. The SMTP client typically initiates a Transmission Control Protocol (TCP) connection to the SMTP server on the well-known port designated for SMTP, port number 25.

SMTP is a delivery protocol only. It cannot pull messages from a remote server on demand. Other protocols, such as the Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP) are specifically designed for retrieving messages and managing mail boxes. However, the SMTP protocol has a feature to initiate mail queue processing on a remote server so that the requesting system may receive any messages destined for it (cf. #Remote Message Queue Starting). POP and IMAP are preferred protocols when a user's personal computer is only intermittently powered up, or Internet connectivity is only transient and hosts cannot receive message during off-line periods.

E-mail is submitted from an message user agent (MUA), the user's email client, to a mail server (MSA), usually using SMTP. From there, the MSA delivers the mail to an MTA, often running on the same machine. These functions may not be distinguished, or merged into one program, and a message may be directly submitted to an MTA: port 587 is used for submission to MSAs (thence to MTAs), while port 25 is used for transferring to MTAs.

The MTA looks up the destination mail exchanger records in the DNS, and relays the mail to the server on record via TCP port 25 and SMTP. Once the receiving MTA accepts the incoming message, it is delivered via a mail delivery agent (MDA) to a server which is designated for local mail delivery. The MDA either delivers the mail directly to storage, or forwards it over a network using either SMTP or the Local Mail Transfer Protocol (LMTP), a derivative of ESMTP 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, the email clients, using 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. Webmail clients may use either method, but the retrieval protocol is often not a formal standard. Some local mail servers and MUAs are capable of either push or pull mail retrieval.

SMTP defines message transport, not the message content. Thus, it defines the envelope, such as the envelope sender, but not the headers or body of the message itself. For example, STD 10 and RFC 5321 define SMTP (the envelope), while STD 11 and RFC 5322 define the message (headers and body), formally referred to as the Internet Message Format (IMF).