13 Facts About SMTP

SMTP's origins began in 1980, building on concepts implemented on the ARPANET since 1971.

SMTP grew out of these standards developed during the 1970s.

SMTP standard was developed around the same time as Usenet, a one-to-many communication network with some similarities.

Original SMTP protocol supported only unauthenticated unencrypted 7-bit ASCII text communications, susceptible to trivial man-in-the-middle attack, spoofing, and spamming, and requiring any binary data to be encoded to readable text before transmission.

ESMTP defines consistent and manageable means by which ESMTP clients and servers can be identified and servers can indicate supported extensions.

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Message submission and SMTP-AUTH were introduced in 1998 and 1999, both describing new trends in email delivery.

Originally, SMTP servers were typically internal to an organization, receiving mail for the organization from the outside, and relaying messages from the organization to the outside.

SMTP is a connection-oriented, text-based protocol in which a mail sender communicates with a mail receiver by issuing command strings and supplying necessary data over a reliable ordered data stream channel, typically a Transmission Control Protocol connection.

Fully capable SMTP servers maintain queues of messages for retrying message transmissions that resulted in transient failures.

Main identification feature for ESMTP clients is to open a transmission with the command EHLO, rather than HELO.

The format of additional SMTP verbs was set and for new parameters in MAIL and RCPT.

An SMTP server that requires a specific capitalization method is a violation of the standard.

Original design of SMTP had no facility to authenticate senders, or check that servers were authorized to send on their behalf, with the result that email spoofing is possible, and commonly used in email spam and phishing.