Simple Mail Transfer Protocol

One of the most attaractive part of the internet is sending and receiving emails. How do we send and receive is still a mystery to many. Simple Mail Transfer Protocol(SMTP) is the standard technology used by people around the globe for sending emails. It was a protocol defined in RFC 821, used to send electronic mail between computers over Ethernet. This protocol is a server to server protocol. To access messages other protocols are used. Clients generally use Internet Message Access Protocol(IMAP) or Post Office Protocol (POP) to access their mail accounts on the mail server.SMTP is the de facto standard.

SMTP sessions initiates by client sending request to the SMTP server. The SMTP server verifies the commands and sends the messages to the recipients after verifying the receiver's address. One the message is send the session is closed.
The SMTP protocols delivers the message, but it cannot recieve the message.

The SMTP specification did not authenticate the sender. This was one of the major limitation. To control spamming and increase security SMPT-AUTH was defined (Read RFC 2554).
RFC 1869 defined the capability for SMTP service extensions, ESMTP or Extended SMTP is defined allowing new service extensions to be defined and registered with IANA. One of the most important extensions available is Delivery Status Notification(DSN),which is defined in RFC 1891
Related RFC
RFC2821

References
RFC 821
RFC 1891
The First Network Email