SMTP is the protocol responsible for sending email between servers. It establishes connections between mail transfer agents and delivers messages hop by hop until they reach their destination. SMTP is text-based and uses clear command and response sequences to coordinate delivery.
It relies on DNS to determine where emails should be sent and how servers identify themselves. SMTP originally trusted every server, so modern systems add layers like TLS, SPF, DKIM, and DMARC to authenticate senders and reduce spam.
SMTP is not designed for retrieving messages, which is why protocols like IMAP and POP3 exist. It works best when networks are stable, but can queue messages for retry during outages. SMTP also supports relays, which forward messages on behalf of others.
Many cloud services now expose SMTP interfaces for sending programmatically. Despite its age, SMTP remains a core building block of global communication.