Simple Mail Transfer Protocol (SMTP) is a protocol for sending e-mail messages between servers. E-mailing systems use this protocol to send mails over the Internet. SMTP works on the application layer of the TCP/IP or OSI reference model. The SMTP client typically initiates a Transmission Control Protocol (TCP) connection to the SMTP server on the well-known port designated for SMTP, port number 25. However, e-mail clients require POP or IMAP to retrieve mails from e-mail servers.
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