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

Authentication

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

Feedback

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