SMTP: The Protocol Behind Email Delivery – What Is It and How Does It Work

Simple Mail Transfer Protocol (SMTP) is an application-layer protocol, introduced in 1982, that defines how email clients and servers send messages across networks through command-and-response exchanges. SMTP transfers message headers, recipients, and content, while MIME extends it to support attachments, non-English text, and HTML formatting; ESMTP adds modern extensions such as EHLO, extra parameters, and SMTP authentication, and SMTPS protects transmissions with SSL/TLS encryption. Email delivery involves clients, SMTP servers, mail agents such as MUAs, MSAs, MTAs, and MDAs, and potentially SMTP relays or gateways that route messages between domains using DNS records. An SMTP session typically uses TCP, begins with HELO or EHLO, sends MAIL FROM, RCPT TO, and DATA commands, and closes with QUIT, while queues hold messages temporarily when receiving servers are unavailable or volume is high. Common SMTP ports include 25 for relay, 587 with STARTTLS for secure submission, and 2525 as an alternative, whereas IMAP and POP3 are separate protocols for retrieving incoming email. SMTP can be integrated into applications using server credentials or obtained through relay services, which may provide delivery infrastructure, analytics, and domain verification.

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