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SMTP
SMTP (Simple Mail Transfer Protocol) is one of the core protocols used for sending emails across the Internet. It facilitates the reliable transfer of outgoing mail between email servers and from email clients to servers, enabling seamless global communication. Operating primarily over TCP port 25, SMTP ensures that messages are relayed efficiently, serving as the foundation of modern Internet email infrastructure..
Simple Mail Transfer Protocol (SMTP) is an application layer protocol defined initially in RFC 821 and updated by RFC 5321. It is designed to transfer mail reliably and efficiently between mail servers. SMTP uses a clear text, command-based communication method that defines how messages are submitted, relayed, and forwarded, primarily functioning in a push model where the client initiates the connection to the server.
By default, SMTP operates over TCP port 25 for server-to-server communications. When an email client submits outgoing mail, it usually connects using port 587 with submission extensions for authentication or port 465 if employing legacy SSL encryption. SMTP dictates the format and delivery of messages but does not retrieve emails from servers; protocols like POP3 or IMAP fulfill that role.
SMTP sessions initiate with a handshake sequence involving HELO/EHLO commands, followed by sender and recipient declarations, and finally the DATA phase where the message content, including headers and body, is transmitted. The connection typically closes upon completion with the QUIT command. Extensions such as ESMTP enhance SMTP's capabilities, adding support for authentication, binary MIME, and more.