Simple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail (email) transmission.1
The objective of the Simple Mail Transfer Protocol (SMTP) is to transfer mail reliably and efficiently.
SMTP is independent of the particular transmission subsystem and requires only a reliable ordered data stream channel. While this document specifically discusses transport over TCP, other transports are possible.2
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A Request for Comments (RFC) is a type of publication from the Internet Engineering Task Force (IETF) and the Internet Society (ISOC), the principal technical development and standards-setting bodies for the Internet.3
- RFC 5321 - Simple Mail Transfer Protocol - The objective of the Simple Mail Transfer Protocol (SMTP) is to transfer mail reliably and efficiently.
- RFC 7504 - SMTP 521 and 556 Reply Codes - This memo defines two Simple Mail Transfer Protocol (SMTP) reply codes, 521 and 556. The 521 code was originally described in an Experimental RFC in 1995 and is in wide use, but has not previously been formally incorporated into SMTP. The 556 code was created to support the new tests and actions specified in RFC 7505. These codes are used to indicate that an Internet host does not accept incoming mail at all. This specification is not applicable when the host sometimes accepts mail but may reject particular messages, or even all messages, under specific circumstances.
- PHP - Swift Mailer integrates into any web app written in PHP, offering a flexible and elegant object-oriented approach to sending emails with a multitude of features.6
- Python - smtplib SMTP protocol client7