Knowledgebase
SMTP Command Reference
Posted by Sean Syed on 18 February 2013 07:44 PM
Overview
Simple Message Transfer Protocol (SMTP) is an asymmetric response protocol. This means that it transmits a command, and then waits for a response before transmitting the next command. The commands that SMTP uses are words, but the responses are numeric codes. Following are some common SMTP commands.
  HELO Sent by a client to identify itself, usually with a domain name.
  EHLO Enables the server to identify its support for Extended Simple Mail Transfer Protocol (ESMTP) commands.
  MAIL FROM Identifies the sender of the message; used in the form MAIL FROM:.
  RCPT TO Identifies the message recipients; used in the form RCPT TO:.
  TURN Allows the client and server to switch roles and send mail in the reverse direction without having to establish a new connection.
  ATRN The ATRN (Authenticated TURN) command optionally takes one or more domains as a parameter. The ATRN command must be rejected if the session has not been authenticated.
  ETRN An extension of SMTP. ETRN is sent by an SMTP server to request that another server send any e-mail messages that it has.
  SIZE Provides a mechanism by which the SMTP server can indicate the maximum size message supported. Compliant servers must provide size extensions to indicate the maximum size message that can be accepted. Clients should not send messages that are larger than the size indicated by the server.
  ETRN An extension of SMTP. ETRN is sent by an SMTP server to request that another server send any e-mail messages that it has.
  PIPELINING Provides the ability to send a stream of commands without waiting for a response after each command.
  CHUNKING An ESMTP command that replaces the DATA command. So that the SMTP host does not have to continuously scan for the end of the data, this command sends a BDAT command with an argument that contains the total number of bytes in a message. The receiving server counts the bytes in the message and, when the message size equals the value sent by the BDAT command, the server assumes it has received all of the message data.
  DATA Sent by a client to initiate the transfer of message content.
  DSN An ESMTP command that enables delivery status notifications.
  RSET Nullifies the entire message transaction and resets the buffer.
  VRFY Verifies that a mailbox is available for message delivery; for example, vrfy ted verifies that a mailbox for Ted resides on the local server. This command is off by default in Exchange implementations.
  HELP Returns a list of commands that are supported by the SMTP service.
  QUIT Terminates the session.
  AUTH The AUTH command is an ESMTP command (SMTP service extension) that is used to authenticate the client to the server. The AUTH command sends the clients username and password to the e-mail server. AUTH can be combined with some other keywords asPLAINLOGINCRAM-MD5 and DIGEST-MD5 (e.g. AUTH LOGIN) to choose an authentication mechanism. The authentication mechanism chooses how to login and which level of security that should be used.
  NOOP The NOOP command does nothing else than makes the receiver to send an OK reply. The main purpose is to check that the server is still connected and is able to communicate with the client.
  STARTTLS STARTTLS is an extension to plain text communication protocols. It offers a way to upgrade a plain text connection to an encrypted (TLS or SSL) connection instead of using a separate port for encrypted communication.
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