SMTP website screenshot

SMTP

Simple Mail Transfer Protocol (SMTP) is the foundational internet standard for transmitting electronic mail across networks. Defined in RFC 5321 (October 2008), SMTP uses a command-response model over TCP port 25 (or 587 for submission, 465 for SMTPS). It defines commands including EHLO, MAIL FROM, RCPT TO, and DATA, along with a comprehensive set of response codes. SMTP works in conjunction with RFC 5322 (Internet Message Format) which defines the structure of email messages.

SMTP publishes 2 APIs on the APIs.io network. Tagged areas include SMTP, Email, Internet Standards, IETF, and Messaging.

The SMTP catalog on APIs.io includes 1 JSON-LD context and 1 Spectral governance ruleset.

🌐 Visit website 📡 Source on GitHub
14.0/100 emerging ▬ flat Agent 3/100 human only Full breakdown ↓
scored 2026-09-14 · rubric v0.22.0
AccessFreemium
2 APIs
SMTPEmailInternet StandardsIETFMessagingProtocolsRFC 5321

Kin Score

Kin Score Kin Score How this is scored →
scored 2026-09-14 · rubric v0.22.0
Composite quality — 14.0/100 · emerging
Contract Quality 2.7 / 25
Developer Ergonomics 0.0 / 20
Access Clarity 3.2 / 20
Operational Transparency 1.0 / 13
Contract Governance 1.2 / 12
Discoverability 5.9 / 10
Agent readiness — 3/100 · human only
Machine-Readable Contract 0 / 18
Agentic Access Contract 0 / 10
Documented Reversibility 0 / 6
MCP Server 0 / 12
Machine-Readable Auth 0 / 10
Idempotency 0 / 9
Stable Error Semantics 0 / 8
Request/Response Examples 0 / 7
Rate-Limit Signaling 7 / 7
Typed Event Surface 0 / 6
Agent Skills 0 / 5
Well-Known Catalog 0 / 4
Consent & Bot Identity 0 / 3
A2A Agent Card 0 / 8
Dry-Run / Simulate Mode 0 / 4
Delegated User Identity 0 / 6
Protected Resource Metadata 0 / 5
Registration Without a Human 0 / 6
Agentic Commerce Well-Known Document 0 / 5
Create-or-Update Ergonomics could not be measured. We hold no machine-readable contract for this provider to read, so there is nothing to measure a write surface against. Excluded rather than scored zero: never-measured and measured-empty are different facts. Publishing an OpenAPI is what makes this facet — and several others — scorable at all.
Improve this rating by publishing the missing artifacts — every area above can be raised, and the full rubric is at apis.io/rating/. Every facet and dimension name above is a link: it opens that measurement's own page — what it means, the exact checks that feed it, how the whole catalog distributes on it, and the providers at the top of it. This rating is computed from github.com/api-evangelist/smtp: open an issue to ask a question, or submit a pull request to add artifacts. Submit an artifact on GitHub — free → Manage your own listing — the Influence plan, $499/mo →

APIs 2

Individual APIs this provider publishes, each with its own machine-readable definition.

RFC 5321 - Simple Mail Transfer Protocol

RFC 5321 is the IETF standard defining the Simple Mail Transfer Protocol. It specifies the client-server protocol used to transmit email across the internet, including the comma...

RFC 5322 - Internet Message Format

RFC 5322 defines the format of electronic mail messages transmitted via SMTP. It specifies the syntax for message headers (From, To, Subject, Date, etc.) and body structure. Wor...

Pricing Plans 1

Published pricing tiers and plan structures.

Smtp Plans Pricing

3 plans

PLANS

Rate Limits 1

Documented rate limits and quota policies.

Smtp Rate Limits

5 limits

RATE LIMITS

FinOps 1

Cost, billing, and metering signals for API financial operations.

Smtp Finops

FINOPS

Semantic Vocabularies 1

JSON-LD contexts and semantic vocabularies used across these APIs.

Smtp Context

34 classes · 0 properties

JSON-LD

Spectral Rules 1

Spectral governance rulesets for linting and validating these APIs.

SMTP API Rules

6 rules · 4 warnings 2 info

SPECTRAL

JSON Schema 2

Standalone JSON Schema definitions for this provider's data models.

SMTP Email Message

11 properties

JSON SCHEMA

SMTP Session

7 properties

JSON SCHEMA

JSON Structure 1

JSON Structure definitions describing this provider's data shapes.

Smtp Message Structure

0 properties

JSON STRUCTURE

Examples 2

Example request and response payloads for these APIs.

Smtp Message Example

4 fields

EXAMPLE

Smtp Session Example

5 fields

EXAMPLE

Security Posture 1

Authentication, domain security, vulnerability disclosure, and trust-center signals.

Smtp Domain Security

TLSv1.3 · HSTS · DNSSEC · DMARC

SECURITY

Resources

Access & Security 1

Authentication, authorization, and security posture

DomainSecurity
DomainSecurity

Company 1

The organization behind the API

Website
Website

Other 7

Properties that don't map to a standard resource type

IETF RFC
IETF RFC
 RFC Editor
RFC Editor
 Wikipedia
Wikipedia
 Related Standard
Related Standard
 Related Standard
Related Standard
 Related Standard
Related Standard
 IETF Working Group
IETF Working Group

Scroll for all 7

Source (apis.yml)

apis.yml Raw ↑ 
aid: smtp
name: SMTP
description: Simple Mail Transfer Protocol (SMTP) is the foundational internet standard for transmitting electronic mail across
  networks. Defined in RFC 5321 (October 2008), SMTP uses a command-response model over TCP port 25 (or 587 for submission,
  465 for SMTPS). It defines commands including EHLO, MAIL FROM, RCPT TO, and DATA, along with a comprehensive set of response
  codes. SMTP works in conjunction with RFC 5322 (Internet Message Format) which defines the structure of email messages.
type: Index
deliveryModel:
  model: unknown
  open_source: false
  commercial: false
  callable_host: false
  label: Delivery model not determined — needs a product licence on record
  confidence: low
  source:
  - none
  generated: '2026-08-28'
  method: derived
accessModel:
  pricing: freemium
  onboarding: unknown
  trial: false
  try_now: false
  public: false
  label: Freemium
  confidence: medium
  source:
  - plans
  generated: '2026-07-22'
  method: derived
image: https://kinlane-images.s3.amazonaws.com/shared/apis-json/icons/smtp.png
tags:
- SMTP
- Email
- Internet Standards
- IETF
- Messaging
- Protocols
- RFC 5321
created: '2025-01-01'
modified: '2026-05-02'
url: https://raw.githubusercontent.com/api-evangelist/smtp/refs/heads/main/apis.yml
specificationVersion: '0.23'
apis:
- aid: smtp:rfc5321
  name: RFC 5321 - Simple Mail Transfer Protocol
  description: RFC 5321 is the IETF standard defining the Simple Mail Transfer Protocol. It specifies the client-server protocol
    used to transmit email across the internet, including the command set, response codes, session lifecycle, and extension
    mechanism (ESMTP). Published October 2008, it obsoletes RFC 2821.
  humanURL: https://datatracker.ietf.org/doc/html/rfc5321
  tags:
  - SMTP
  - RFC
  - IETF
  - Email Protocol
  - Standards
  properties:
  - type: Specification
    url: https://datatracker.ietf.org/doc/html/rfc5321
  - type: RFC Editor
    url: https://www.rfc-editor.org/rfc/rfc5321.html
  - type: IETF Datatracker
    url: https://datatracker.ietf.org/doc/rfc5321/
- aid: smtp:rfc5322
  name: RFC 5322 - Internet Message Format
  description: RFC 5322 defines the format of electronic mail messages transmitted via SMTP. It specifies the syntax for message
    headers (From, To, Subject, Date, etc.) and body structure. Works in conjunction with RFC 5321.
  humanURL: https://datatracker.ietf.org/doc/html/rfc5322
  tags:
  - Email Format
  - RFC
  - IETF
  - Message Headers
  - Standards
  properties:
  - type: Specification
    url: https://datatracker.ietf.org/doc/html/rfc5322
common:
- type: Website
  url: https://www.ietf.org/
- type: DomainSecurity
  url: security/smtp-domain-security.yml
- type: IETF RFC
  url: https://datatracker.ietf.org/doc/html/rfc5321
- type: RFC Editor
  url: https://www.rfc-editor.org/rfc/rfc5321.html
- type: Wikipedia
  url: https://en.wikipedia.org/wiki/Simple_Mail_Transfer_Protocol
- type: Related Standard
  url: https://datatracker.ietf.org/doc/html/rfc5322
- type: Related Standard
  url: https://datatracker.ietf.org/doc/html/rfc3463
- type: Related Standard
  url: https://datatracker.ietf.org/doc/html/rfc4409
- type: IETF Working Group
  url: https://datatracker.ietf.org/wg/emailcore/
maintainers:
- FN: Kin Lane
  email: kin@apievangelist.com

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