SMTP is short for Simple Mail Transfer Protocol and it is used to transfer e-mail messages between computers. It is a text based protocol in which message text is specified along with the recipients of the message. Simple Mail Transfer Protocol is a 'push' protocol and it cannot be used to 'pull' the messages from the server.

A procedure of queries and responses is used to send the message between the client and the server. An end user's e-mail client or the relaying server's Mail Transport Agents can act as an SMTP client which is used to initiate a TCP connection to the port 25 of the server. SMTP is used to send the message from the mail client to the mail server and an e-mail client using the POP or IMAP is used to retrieve the message from the server.

SMTP Functions

An SMTP server performs the following two functions :

  1. It verifies the configuration and grants the permission to a computer that is making an attempt to send a message. It sends the message to the specified destination and tracks it to see whether it's delivered successfully or not. If it's not delivered successfully then an error message is send to the sender.
  2. There's one limitation to SMTP and it's the inability to authenticate the senders and this results in e-mail spamming. The enhanced version of SMTP also exists and it's called as Extended SMTP (ESMTP). ESMTP is used to send the e-mails that include graphics and other attachments.