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<p><b>New page</b></p><div>[[File:SMTP-transfer-model.svg|thumb|SMTP-transfer-model]] '''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 [[RFC 5321]], which is the current version of the protocol. SMTP is used to send and receive email messages between [[mail servers]] and from [[email clients]] to mail servers.<br />
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==History==<br />
SMTP was developed as part of the [[ARPANET]] project, which was the precursor to the modern [[Internet]]. It was designed to facilitate the transfer of email messages between different systems. The protocol has evolved over time, with various extensions and updates to improve its functionality and security.<br />
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==Functionality==<br />
SMTP operates over the [[Transmission Control Protocol]] (TCP), typically using [[port number]] 25. The protocol follows a client-server model, where the client sends email messages to the server, which then forwards them to the recipient's mail server. SMTP uses a series of commands and responses to facilitate the transfer of messages.<br />
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===Commands===<br />
Some of the key SMTP commands include:<br />
* '''HELO''' or '''EHLO''': Initiates the conversation between the client and server.<br />
* '''MAIL FROM''': Specifies the sender's email address.<br />
* '''RCPT TO''': Specifies the recipient's email address.<br />
* '''DATA''': Indicates the start of the message content.<br />
* '''QUIT''': Terminates the session.<br />
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===Responses===<br />
SMTP servers respond to commands with numerical codes and text messages. Common response codes include:<br />
* '''250''': Requested mail action okay, completed.<br />
* '''354''': Start mail input; end with <CRLF>.<CRLF>.<br />
* '''421''': Service not available, closing transmission channel.<br />
* '''550''': Requested action not taken: mailbox unavailable.<br />
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==Security==<br />
SMTP was not originally designed with security in mind, which has led to various vulnerabilities and issues such as [[spam]] and [[email spoofing]]. To address these concerns, several extensions and protocols have been developed, including:<br />
* [[STARTTLS]]: Provides encryption for SMTP connections.<br />
* [[SMTP Authentication]] (SMTP AUTH): Requires users to authenticate before sending email.<br />
* [[DomainKeys Identified Mail]] (DKIM) and [[Sender Policy Framework]] (SPF): Help verify the authenticity of email messages.<br />
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==Related Protocols==<br />
SMTP is often used in conjunction with other email protocols, such as:<br />
* [[Post Office Protocol]] (POP3): Used by email clients to retrieve messages from a mail server.<br />
* [[Internet Message Access Protocol]] (IMAP): Allows email clients to access and manage email messages on a mail server.<br />
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==See Also==<br />
* [[Email]]<br />
* [[Mail server]]<br />
* [[Transmission Control Protocol]]<br />
* [[Internet standard]]<br />
* [[RFC 5321]]<br />
* [[STARTTLS]]<br />
* [[SMTP Authentication]]<br />
* [[DomainKeys Identified Mail]]<br />
* [[Sender Policy Framework]]<br />
* [[Post Office Protocol]]<br />
* [[Internet Message Access Protocol]]<br />
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==References==<br />
{{Reflist}}<br />
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==External Links==<br />
{{Wikibooks|Simple Mail Transfer Protocol}}<br />
{{Wiktionary|SMTP}}<br />
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[[Category:Internet protocols]]<br />
[[Category:Email]]<br />
[[Category:Application layer protocols]]<br />
[[Category:Internet Standards]]<br />
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