SMTP

The Simple Mail Transfer Protocol (SMTP) is the application-layer protocol (RFC 5321) governing all email transmission between Mail Transfer Agents (MTAs), operating primarily over TCP ports 25, 587, or 465.

SMTP is the foundational protocol (RFC 5321) for transporting electronic mail across the internet: it manages the delivery envelope, not the message content itself. A client initiates a TCP connection with an SMTP server, using a command sequence—specifically `EHLO`, `MAIL FROM`, `RCPT TO`, and `DATA`—to transfer the message. Server-to-server relaying uses the traditional port 25; authenticated client submission uses port 587 (Submission) with STARTTLS, or sometimes port 465 (SMTPS). This client-server architecture, executed by Message Transfer Agents (MTAs) like Postfix or Sendmail, ensures reliable, ordered message delivery from sender to final recipient.

https://datatracker.ietf.org/doc/html/rfc5321