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What is SMTP?
SMTP (Simple Mail Transfer Protocol)
is a protocol that uses the TCP/IP protocol and is used to send and receive emails. However, because of its limited capacity to queue messages on the receiving end, it is often used with one of the other two protocols, POP3 (Post Office Protocol) or IMAP (Internet Message Access Protocol), allowing the user to save their messages to a server mailbox and periodically download them from the server. In other words, users typically use an intermediary program that uses SMTP to send e-mail and POP3 or IMAP protocols to receive e-mail.
How SMTP Works
SMTP works in a three-step process using the client server model. First, an email server uses SMTP to send a message from the email client. Outlook or Gmail might be an example of an email server. Second, the e-mail server uses SMTP as a relay service to send e-mail to the receiving server. Third, the receiving server uses an email client to download incoming mail via IMAP and place it in the recipient's inbox.
Differences Between SMTP and IMAP
SMTP hosted on an SMTP server is used to send or forward messages from a mail client, but cannot receive messages.
IMAP, on the other hand, is an email protocol that deals with managing and receiving email messages. IMAP keeps an email on the server and then synchronizes it across several devices. IMAP is used to retrieve emails, not send.
When used together, SMTP and IMAP receive and forward email messages.
SMTP and Unix-Based Systems
On Unix-based systems, sendmail remains the most widely used SMTP mail server for sending e-mail. Sendmail enables email transfer in the background and is usually used with a separate user-friendly interface.
SMTP is typically implemented to work using Internet Port 25. An alternative to SMTP, which is widely used in Europe, is X.400/X.500. When it was created, X.400 was intended to serve as the primary international protocol.