Simple Mail Transfer Protocol (SMTP) is used to send mail across the internet. SMTP protocol pushes messages from one server to another. A Message transfer agent is used to pass mail from the sending machine to the receiving machine. The MTA on both machines use the network SMTP (simple mail transfer protocol) to pass mail between them, usually on port 25.

RFC 821, RFC 1854 RFC 2821, RFC 2822 apply to SMTP. RFC 1869, RFC 1891-1895, and RFC 3463 apply to Extended SMTP.

SMTP Commands include:

  1. HELO - Sent by client with domain name such as
  2. MAIL - From <>
  3. RCPT - To <>
  4. DATA - Sends the contents of the message. The headers are sent, then a blank line, then the message body is sent. A line with "." and no other characters indicates the end of the message.
  5. QUIT