SMTP
also: Simple Mail Transfer Protocol
SMTP (Simple Mail Transfer Protocol) is the standard protocol used to send emails from a client to a mail server or between mail servers across the internet.
SMTP is a text-based protocol that operates over TCP port 25 (or ports 587 and 465 for authenticated connections). When you send an email, your mail client connects to an SMTP server and transmits the message using a series of simple commands like MAIL FROM, RCPT TO, and DATA.
Unlike IMAP or POP3 (which retrieve emails), SMTP is strictly for sending. For example, when you click "Send" in Gmail or Thunderbird, your message goes through SMTP to reach the recipient's mail server.
Modern SMTP often uses authentication (SMTP AUTH) and encryption (TLS/SSL) to secure credentials and message content in transit. Mail servers themselves use SMTP to relay messages to other servers across the internet.