# RFC 5321 - Simple Mail Transfer Protocol

**Canonical:** https://apis.io/apis/smtp/rfc5321/  
**Provider:** SMTP — https://apis.io/providers/smtp/  
**Documentation:** https://datatracker.ietf.org/doc/html/rfc5321

RFC 5321 - Simple Mail Transfer Protocol is one of 2 APIs that [SMTP](https://apis.io/providers/smtp/) publishes on the [APIs.io](https://apis.io/) network. Tagged areas include SMTP, RFC, IETF, Email Protocol, and Standards.

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.

## Machine-readable artifacts (4)

- **Specification** — https://datatracker.ietf.org/doc/html/rfc5321
- **RFC Editor** — https://www.rfc-editor.org/rfc/rfc5321.html
- **IETF Datatracker** — https://datatracker.ietf.org/doc/rfc5321/
- **APIsJSON** — https://raw.githubusercontent.com/api-evangelist/smtp/refs/heads/main/apis.yml

## Other SMTP APIs (1)

- [RFC 5322 - Internet Message Format](https://apis.io/apis/smtp/rfc5322/)

## Tags

SMTP, RFC, IETF, Email Protocol, Standards

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Profiled by [API Evangelist](https://apievangelist.com) and published on [APIs.io](https://apis.io/apis/smtp/rfc5321/). The API's provider profile, Kin Score and agent-readiness rating are at https://apis.io/providers/smtp/.