SMTP is an essential component of the rules that define how information is transmitted across the internet. This article discusses the history of the protocol and the benefits it provides. It also discusses services that use SMTP to deliver email.

The Simple Mail Transfer Protocol, better known as SMTP, is a protocol used to transmit email messages over the internet. The standard itself was first published in 1981, and it has been improved continuously to adapt to changes in technology and user needs. As an internet standard for over 35 years, SMTP has been adopted universally as the protocol of choice for transmitting email from one email server to another.

The SMTP standard defines the conversation between the sender of an email and the SMTP mail server that delivers the email. In an SMTP conversation, the sender issues a certain sequence of commands to the receiving SMTP server. Generally, if the sender issues valid commands, and the intended recipients of the email are valid accounts on the receiving server, the receiving server will accept the message and attempt to deliver it. Of course, the vast majority of email senders never see this conversation in action, because their email clients handle these transactions behind the scenes.

  • Internet Engineering Task Force (IETF) – An international organization that creates standards for communicating information over the internet. The protocols that make the World Wide Web and email possible were created and promoted by the IETF.
  • Mail client (or email client) – An application used to compose, send, and receive email on a user's computer or mobile device.
  • Multipurpose Internet Mail Extensions (MIME) – An internet standard that expands the capabilities of email sent via SMTP. The original SMTP specification only supported unaccented Latin characters in the body of the email, and did not support non-text attachments. The MIME standard allows for non-Latin characters, non-text attachments, and emails that contain multiple parts (such as a plain text component and an HTML component).
  • Open mail relay – An SMTP server that allows anyone on the internet to send email through it, without having to authenticate (such as by providing a user name or password). SMTP servers were originally all open relays, but spammers and other malicious users soon started to abuse them. With very few exceptions, all SMTP servers now require some sort of authentication.
  • SMTP Authentication – An extension of the original SMTP protocol that provides methods for email clients to authenticate when connecting to an SMTP server, such as by providing a user name and password. Using SMTP Authentication prevents unauthorized users, such as spammers, from sending email through an SMTP server.
  • STARTTLS – An extension to plain text communication protocols (such as SMTP) that provides methods for encrypting connections that use those protocols. Encryption effectively prevents information sent over these connections from being intercepted by third parties.

Because SMTP is an open standard, any application can use the protocol to transmit email from a client to a server. The simple nature and thoroughly-defined rules governing the protocol make it extremely reliable and predictable. At the same time, the protocol was built to allow a great deal of flexibility. For example, the original SMTP specification was designed only to send unaccented Latin text, making it less useful for those who use languages with accented Latin characters, or non-Latin alphabets. And because the protocol has been adopted universally, there are no compatibility issues when sending messages between SMTP servers.

Amazon Simple Email Service (Amazon SES) provides an SMTP interface that you can use to send email from applications you develop. For example, if you have already built email sending functions into your application, you can use the SMTP relay to send those messages using Amazon SES. You can also use the SMTP relay to deliver messages generated by third-party applications. For example, if your business uses a ticketing system, you can configure that system to use Amazon SES to deliver the emails it generates.

It's easy to get started with Amazon SES. Follow the Getting Started Guide and start sending email in minutes.

Amazon Web Services users who call Amazon SES from an application hosted in an Amazon EC2 instance can send up to 62,000 emails per month at no additional charge. To learn more, see Pricing.

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