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.

2 APIs 0 Features

SMTPEmailInternet StandardsIETFMessagingProtocolsRFC 5321

APIs

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

Smtp Plans Pricing

3 plans

PLANS

Rate Limits

Smtp Rate Limits

5 limits

RATE LIMITS

FinOps

Smtp Finops

FINOPS

Semantic Vocabularies

Smtp Context

34 classes · 0 properties

JSON-LD

JSON Structure

Smtp Message Structure

0 properties

JSON STRUCTURE

Example Payloads

Smtp Message Example

4 fields

EXAMPLE

Smtp Session Example

5 fields

EXAMPLE

Resources

IETF Working Group

IETF Working Group

Sources

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
image: https://kinlane-images.s3.amazonaws.com/shared/apis-json/apis-json-logo.jpg
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: '0.19'
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: 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