Internet Protocols and Standards


The Internet Engineering Task Force (IETF) sets standards and guidelines with formal documents called Requests for Comment (RFCs). All email works according to the protocols and frameworks described in these documents, and they can be very useful reference material for understanding delivery success and failure.

RFC Requests For Comment

SMTP – Simple Mail Transfer Protocol

RFC 5321 Simple Mail Transfer Protocol
How email is sent over the wire
RFC 5322 Internet Message Format
The structure of email messages, especially headers

MIME – Multimedia Internet Email Extensions

RFC 2045 – Multipurpose Internet Mail Extensions (MIME) Part One
The basics of creating a MIME message, headers and encoding
RFC 2046 – Multipurpose Internet Mail Extensions (MIME) Part Two
Different media types – text, images, etc – and multipart messages, needed for rich text and attachments
RFC 2047 – MIME Part Three: Message Header Extensions for Non-ASCII Text
Internationalized text or glyphs in subject lines or friendly from


RFC 6376 – DomainKeys Identified Mail (DKIM) Signatures
DKIM, a way to allow a sender to prove they’re responsible for a message
RFC 4408 – Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1
SPF, aka Sender Permitted From, aka Sender Policy Framework


RFC 5965 – An Extensible Format for Email Feedback Reports
Sending automated reports when a user hits the “this is spam” button
RFC 6430 – Email Feedback Report Type Value: not-spam

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