Glossary

SMTP (Simple Mail Transfer Protocol)

SMTP is the protocol every email server speaks to every other email server. It is from 1982 (RFC 821), updated periodically (most recently RFC 5321). When you call a transactional API to send mail, that API's MTA opens an SMTP connection to the recipient's MTA on port 25, transfers the message envelope and body, and the recipient takes responsibility.

One-line definition

SMTP is the text-based, line-oriented protocol mail servers use to relay messages to each other. It is the lingua franca of email delivery — every message in transit, regardless of the sender API, eventually crosses SMTP between MTAs.

The basic SMTP conversation

A complete envelope handshake is four commands and one body. Servers respond with three-digit reply codes (2xx success, 4xx temporary failure, 5xx permanent failure).

S: 220 mta.example.com ESMTP
C: HELO sender.example.org
S: 250 Hello sender.example.org
C: MAIL FROM:<alice@example.org>
S: 250 OK
C: RCPT TO:<bob@example.com>
S: 250 OK
C: DATA
S: 354 End data with <CRLF>.<CRLF>
C: From: alice@example.org
C: To: bob@example.com
C: Subject: Hello
C: 
C: Body of the message
C: .
S: 250 OK queued as 4f8a3
C: QUIT
S: 221 Bye

The envelope vs the headers

SMTP has two notions of who sent / received a message: the envelope (the MAIL FROM and RCPT TO commands above) and the headers (the From: and To: lines inside the body).

These can differ — and often do, deliberately. A forwarder rewrites the envelope sender to itself but leaves the header From: untouched. An automated send might use a bounce@yourdomain.com envelope sender to route bounces into a process that handles them, while the user sees hello@yourdomain.com in From:. Authentication mechanisms (SPF, DMARC) care about both.

Ports: 25, 465, 587, 2525

25 — Server-to-server delivery. Most ISPs block outbound 25 on residential / cloud-VM networks to suppress spam from compromised hosts. Always cleartext-by-default; STARTTLS upgrade is optional but ubiquitous.

465 — Implicit-TLS submission. Originally non-standard, now formally defined in RFC 8314. Used by mail clients submitting to their provider.

587 — Submission port (RFC 4409). The standard port for authenticated mail clients to submit outbound mail. Cleartext-by-default with mandatory STARTTLS upgrade.

2525 — Unofficial alternative when 25/587 are blocked. Many providers (SNDR included) offer SMTP on 2525 for serverless / cloud users whose providers block the standard ports.

STARTTLS and opportunistic encryption

STARTTLS is the SMTP command that upgrades a cleartext connection to TLS. Most server-to-server SMTP is opportunistic — both sides offer STARTTLS, both sides upgrade if able, but plaintext is still acceptable as a fallback.

MTA-STS (RFC 8461) tightens this by letting domains publish a policy saying "always require TLS when sending mail to me" — a defence against active downgrade attacks. TLS-RPT (RFC 8460) sends reports back when STARTTLS negotiation fails.

When to use SMTP vs an HTTP API

HTTP APIs (POST /v1/send) are easier to integrate, give structured error codes, support idempotency keys, and work on every serverless platform — they are the default.

SMTP is the right choice when you need byte-perfect control over the message body (specific MIME structure, custom headers, exotic encodings), when integrating legacy software that only speaks SMTP, or when a serverless host blocks outbound HTTP to email APIs.

SNDR speaks both. The HTTP API is recommended for greenfield code; the SMTP relay is available for the cases the API does not cover.

Related

Deliverability

What happens between SMTP accepting your message and the inbox.

Read

Bounce rate

The SMTP 5xx half of deliverability metrics.

Read

Webhooks

How SNDR reports SMTP-level events back to your app.

Read