Simple Mail Transfer Protocol Server is a TCP/IP (Transmission Control Protocol/Internet Protocol) used in sending and receiving/accepting emails. In any case, since it is restricted in its ability to queue messages at less than desirable ends, it is usually one of two different protocols, POP3 (Post Office Protocol) or IMAP (Internet Message Access Protocol) used together, Users save messages to the server mailbox and download them from the server occasionally. At the end of the day, users typically use a program that uses SMTP to send email and POP3 or IMAP to receive email.
SMTP is only a delivery protocol. In common use, mail destination is “pushed” to the mail server. Mail is routed based on the destination server and not the individual user (s) to whom it is addressed. Other protocols, such as Post Office Protocol (POP) and Internet Message Access Protocol (IMAP) are used exclusively to manage messages and mailboxes used by individual users. To allow intermittently connected mail servers to pull messages from a remote server on demand, the SMTP server has the facility to initiate mail queue processing on the remote server. POP and IMAP are inappropriate protocols for relaying mail by inter-connected machines.
Remote message queue initialization is a feature of SMTP servers that allows a remote host to begin processing a mail queue on a server. Then it can receive the messages it receives by sending the TURN command. However, this feature is considered unsafe and has been extended in RFC 1985 with the ETRN command that operates more securely using an authentication method based on domain name system information.
ODMR, also known as authenticated TURN (ATRN). Therefore it allows a user to connect to an ISP using a dynamic IP address, authenticate and request Email.