Simple Mail Transport Protocol a protocol governing email transmission and reception
(sending and receiving emails between mail servers).
A typical example of how two MTAs communicate with each other using a single TCP connection.
Below "The session" describing a one-line email message sent by using SMTP Simple Mail Transport Protocol governing email transmission and reception?
The MTA used for this session is Sendmail--a software program for sending emails.
-V MTA sent/received across the SMTP connection 220 a 3 digit numeric reply code from the SMTP server
sun % mail
Sender type mail to invoke sender user agent.
From: Command identifies the message
>>> QUIT Command, terminates mail exchange.
QUIT command, the receiving machine must send an OK reply to the sending machine--at this point--the receiving machine do not close the transmission yet--until it receives and reply to the QUIT command from the sender machine. The sender machine will also not close the transmission until it sends the QUIT command and receives a reply.
If the connection is closed prematurely, a 504 command not implemented failure reply to the sending machine and the receiving machine will act as if a RSET command had been received, cancel pending transaction due to temporary error. A RSET command specifies that current mail transaction is to be aborted and the receiver machine must send an OK reply.
RSET command aborts current mail transaction and causes both ends to reset and any stored information about sender, recipients or mail data is discarded. A mail transaction may be aborted by the RSET command.
If the transaction beginning command argument is not acceptable a 501 failure reply must be returned. If the commands in a transaction are out of order a 503 failure reply must be returned. See SMTP Syntax error codes
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Basically, email consist of:
(1) The envelope
(3) Message body