SHORT ANSWER
“Simple Mail Transfer Protocol” (Popularly known as SMTP) refers to the protocol for sending email messages from one server to another on a TCP or IP network. SMTP is also known as “RFC 821” and “RFC 2821”. “Simple Mail Transfer Protocol” (SMTP) is created and maintained by the “Internet Engineering Task Force” (IETF).

Mostly the email systems which send messages over the Internet using the “SMTP” between the servers with the emails that can be later retrieved with the help of an email client over “POP3” or “IMAP”. Moreover, SMTP is most commonly used to send mails from a mail client to a mail server. Thus, when the server configures your email application the specification is needed of both “POP3” or “IMAP” server plus the SMTP.

As we know, “Simple Mail Transfer Protocol” is a “TCP” or “IP” protocol that is used to send and receive emails but because of its limited ability to queue up messages at the receiving end, SMTP usually uses one of two other protocols. These protocols are “POP3” or “IMAP”, which enable the user to save emails in a server mailbox and also periodically download them from the server.

It also means that the users typically make use of a program that uses “SMTP” for sending email and either via “POP3” or “IMAP” for receiving email messages. On UNIX based operating systems, the “Sendmail” is the most widely used “SMTP Server” for email messages. Sendmail is typically acommercial package that includes a “POP3 Server”. Also, “Microsoft Exchange” can be set up to get the “POP3” support as it includes an “SMTP Server”.

SMTP is most commonly implemented to smoothly operate over the “Internet Port 25”. However, the SMTP alternative known as “X.400” is widely used in the European region. Other than SMTP, these days email servers also provide support for “Extended Simple Mail Transfer Protocol” (Popularly known as ESMTP), which makes it easy to deliver multimedia files as an email.