What is SMTP?
Simple Mail Transfer Protocol (SMTP) is an Internet Norm for transferring Electonic Mail (E-mail). It was first conceptualized by Request For Comments (RFC). Request for Comments (RFC) is a form of publication from the Internet Engineering Task Force (IETF) and the Internet Society (ISOC), the primary technical development and standards-setting bodies for the Internet. User-level client mail applications typically use SMTP Server only for sending messages to a mail server for relaying. For retrieving messages, client applications usually use either IMAP or POP3.
SMTP Server vs Mail retrieval
SMTP is a delivery protocol only. In normal use, mail “pushed” to a destination mail server as it arrives. Mail is routed based on the destination server, not the individual user(s) to which it is addressed. Other protocols, such as the Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP) specifically designed for use by individual users retrieving messages and managing mailboxes. To permit an intermittently-connected mail server to pull messages from a remote server on demand, SMTP Server has a feature to initiate mail queue processing on a remote server. POP and IMAP are unsuitable protocols for relaying mail by intermittently- connected machines.
Remote Message Queue Starting
Remote Message Queue Starting is a feature of SMTP Server that permits a remote host to start processing of the mail queue on a server. So it may receive messages destined to it by sending the TURN command. This feature, however, deemed insecure and extended in RFC 1985 with the ETRN command which operates more securely using an authentication method based on Domain Name System information.
On-Demand Mail Relay
ODMR, also known as Authenticated TURN (ATRN). So It allows a user to connect to an ISP, authenticate, and request e-mail using a dynamic IP address.