Simple Mail Transfer Protocol can be shortened as SMTP, which is a protocol used for sending email over the internet. SMTP is an internet standard for electronic mail (e-mail) transmission. It was firstly defined by RFC 821 in 1982, and was last updated in 2008 with the Extended SMTP additions by RFC 5321, which is a protocol widely used today. In general, SMTP is a set of commands that authenticate and validate the direct transfer of electronic mail. SMTP by default uses TCP port 25, whereas the protocol for mail submission is the same, but uses port number 587. SMTP connections are secured by SSL, called as SMTPS, connection is default to port to 465 (nonstandard, but used for legacy concerns). Although, electronic mail servers and other mail transfer agents use SMTP to send/ receive emails, user-level clients typically use a program that uses SMTP for sending e-mail to a mail server for relaying. For receiving messages, client-side applications use POP3 or IMAP.
Simple-mail-transfer-protocol is an application layer protocol. SMTP fundamentally is always on the listening mode. So as soon as it listens for a TCP connection form any client and the very soon the TCP connection is made the client process is able to send the mail the very instant.
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