Simple Mail Transfer Protocol (SMTP) is the standard Internet protocol used to transport email between mail systems. It handles sending and relaying messages; separate standards define the message’s format, and POP or IMAP are commonly used to access received mail.
What does SMTP do?
SMTP moves email from one mail system to another. A message may pass through intermediate relay or gateway hosts before reaching its destination. As RFC 5321 puts it, “The objective of the Simple Mail Transfer Protocol (SMTP) is to transfer mail reliably and efficiently.” The specification covers transport and relaying, as well as uses for submitting mail; many configurations use a separate message-submission protocol for the latter. RFC 5321
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How does an SMTP transaction work?
After a client connects, the receiving server sends a greeting. The client typically responds with EHLO, identifying itself and asking which SMTP extensions the server supports. HELO is a backward-compatible alternative that does not request extensions. The client and server exchange replies as the transaction proceeds, allowing the client to track whether commands were accepted. RFC 5321
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MAIL FROMidentifies the sender or return path for the SMTP envelope.#1 Best Overall
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RCPT TOidentifies a recipient. A transaction can include this command more than once for multiple recipients. -
DATAbegins transmission of the message content. The client ends that transmission with SMTP’s end-of-data marker.Rank #2
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The envelope commands route the mail; the content sent after DATA carries the message. The envelope sender and recipients serve a transport purpose and should not be confused with the message’s visible headers. RFC 5321
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Is SMTP the same as the email message itself?
No. SMTP transports the message, but it does not define the full structure of its headers and body. RFC 5322 specifies the Internet Message Format, including message headers and bodies. In short, SMTP moves mail; message-format rules describe how the mail is structured.
| Protocol or standard | Main purpose |
|---|---|
| SMTP (RFC 5321) | Transporting and relaying email between mail systems |
| RFC 5322 | Defining the structure of message headers and bodies |
| POP and IMAP | Accessing or retrieving mail, rather than transporting it between systems |
The exact arrangement depends on the task: an email app may submit outgoing mail to a provider, while mail servers relay messages between domains. SMTP is not a replacement for POP or IMAP when the need is to retrieve or access received mail. RFC 5321
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Does SMTP encrypt email?
SMTP by itself does not guarantee an encrypted connection. A server and client can negotiate TLS using the STARTTLS extension, but ordinary STARTTLS is opportunistic: if TLS is not negotiated, mail may otherwise be sent without transport encryption. Interference with negotiation and the lack of server authentication are among the risks described in RFC 8689.
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REQUIRETLS provides an additional option for requiring TLS-protected transmission when the relevant servers support and honor it. That requirement does not guarantee encryption across every possible route on the public Internet: support and enforcement must hold along the delivery path. RFC 8689
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