Full Form of SMTP:
Simple Mail Transfer Protocol
SMTP Full Form is Simple Mail Transfer Protocol. SMTP is a standard internet protocol that is used for sending and receiving e-mail messages between two or more servers. Most of the e-mail systems, working over the internet, use SMTP to send messages from one server to the other. This protocol generally sends e-mail messages from the source e-mail client to the recipient e-mail server for relaying.
Usually, POP3(Post Office Protocol 3) or IMAP (Internet Message Access Protocol) is used by the client applications to receive messages. This protocol was first defined in 1982 by the help of RFC 821, and the last and latest update took place in 2008. TCP port 25 is the default port for SMTP. MAIL, RCPT, and DATA are the three command or reply sequences that are mostly and typically used in a SMTP transaction. But this protocol does not have a strong user authentication feature.
Thus, SMTP users can experience email phishing, spam or email spoofing. At the receiving end, SMTP’s ability to queue messages is limited. A nonstandard default port 465 is used in SMTP for maintaining security through SSL. Most of the mail servers of this era use ESMTP (Extended SMTP) in order to send or deliver multimedia files as e-mail messages.