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Backend Development › Email & Notifications

SMTP

The protocol for sending email between servers.

Backend Engineer · Mid-level

Also known as: SMTP, simple mail transfer protocol, smtp server

SMTP (Simple Mail Transfer Protocol) is the standard for sending and relaying email. When your application sends a message, it hands it to an SMTP server (a relay), which connects to the recipient’s mail server and delivers it. The recipient later reads it via a different protocol (see IMAP/POP3).

app → SMTP relay → recipient's mail server → recipient's mailbox

Key pieces: the envelope (the actual sending/recipient addresses used for delivery, separate from the visible “To”/“From” headers) and the relay’s credentials/auth. In practice, most applications don’t run their own SMTP server; they use a transactional email provider’s SMTP endpoint or API (see sending email).

The classic mistakes:

  • Running your own SMTP server to send bulk mail. Deliverability from a self-run server is hard (reputation, IP warmup, blocklists); a transactional provider handles it. Run your own only for specific needs.
  • Confusing sending (SMTP) with reading (IMAP/POP3). SMTP sends; IMAP/POP3 retrieve from a mailbox. Mixing them up leads to wrong integrations.
  • Ignoring authentication and port/TLS choices. Modern relays require authentication and an encrypted connection; using the wrong port or no TLS gets rejected or leaks credentials.
  • Trusting the visible “From”. SMTP delivers based on the envelope; spoofing the visible From is easy without SPF/DKIM/DMARC. Authenticate your domain.
  • No bounce handling. The relay reports bounces; ignoring them means sending to dead addresses and damaging reputation (see email deliverability).
  • Blocking the request on SMTP send. Sending email synchronously in a request ties up the user and can fail on relay slowness; queue it (see job queue).

How to use it: for most apps, connect to a transactional email provider over SMTP (or its API), authenticate and encrypt the connection, queue sends rather than doing them inline, and handle bounces. SMTP is the transport; deliverability (authentication, reputation, list hygiene) is the harder part layered on top. See sending email.