SMTP (Simple Mail Transfer Protocol) is the internet standard for sending and relaying electronic mail between servers. Defined in the RFC series and in wide use since the early 1980s, it remains the backbone protocol for email transmission across the internet.
SMTP
SMTP (Simple Mail Transfer Protocol) is the internet standard for sending and relaying electronic mail between servers. First specified in the early 1980s and modernized through RFC 5321, it is one of the oldest application protocols still doing daily work at internet scale. When a message leaves a mail client and hops from server to server toward its destination, SMTP is the language those servers speak.
- Store-and-forward - Mail is relayed server to server, each hop accepting responsibility for delivering onward.
- Text-based commands - A small, human-readable command set (HELO, MAIL FROM, RCPT TO, DATA) that made it easy to implement everywhere.
- Sending, not fetching - SMTP moves mail toward a mailbox; retrieval is left to companion protocols like POP and IMAP.
- Layered security - STARTTLS, SPF, DKIM, and DMARC were added over time to authenticate senders and encrypt transport.
SMTP is one of the protocols I point to when explaining that APIs did not invent machine-to-machine communication — email infrastructure was doing programmatic message exchange decades earlier. In modern API operations it shows up as the transport behind transactional-email and notification services, which many providers now wrap in clean REST APIs so developers never touch the raw protocol. It remains a working example of how a simple, well-documented standard can outlast the systems built on top of it.
Referenced on the API Evangelist blog
Where this standard shows up across sixteen years of my writing at apievangelist.com — how it fits into API design, governance, and the agentic turn.
