Various forms of one-to-one electronic messaging were used in the 1960s. Users communicated using systems developed … See more

Email is submitted by a mail client (mail user agent, MUA) to a mail server (mail submission agent, MSA) using SMTP on TCP port 587. Most mailbox providers still allow submission on traditional port 25. The MSA delive… See more

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 cha… See more

An email client needs to know the IP address of its initial SMTP server and this has to be given as part of its configuration (usually given as a DNS name). This server will deliver outgoing messages on behalf of the user. … See more

A typical example of sending a message via SMTP to two mailboxes (alice and theboss) located in the same mail domain (example.com) is reproduced in the following session exchange. (In this example, th… See more

Clients learn a server's supported options by using the EHLO greeting, as exemplified below, instead of the original HELO. Clients fall back to HELO only if the server does not support EHLO greeting.
Modern clie… See more

The 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 … See more