Fifteen chapters, one mission

01

The History of Email — From ARPANET to DMARCbis

How email went from an unauthorized weekend side project in 1971 to a five-protocol authentication system in 2026 — the full 55-year history, chapter by chapter.

ARPANET RFC 822 SMTP MIME DMARCbis

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02

How Email Travels — From Click to Inbox

The full SMTP delivery path, step by step — from your MUA to the recipient's mailbox, with every server and header explained along the way.

SMTP Received headers MTA / MSA Bounce

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03

Envelope vs Header Addresses — The Two Faces of Email Addressing

Every email carries two sender addresses, not one. Here's what envelope and header addresses do, why they differ, and why DMARC alignment depends on it.

Return-Path MAIL FROM DMARC alignment

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04

Email Message Structure — Headers, Body, and Everything Between

The anatomy of an email message — how the header section is built, what header folding does, and the one blank line that separates headers from body.

Header folding Message body RFC 5322

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05

All Standard Headers — Complete Reference

Every standard email header field defined — From, To, Received, Message-ID, References, and everything RFC 5322 and its companions specify — in one reference.

Header reference RFC 5322 Message-ID

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06

Email Authentication — SPF, DKIM, DMARC, ARC, and BIMI

SPF, DKIM, DMARC, ARC, and BIMI explained as one connected system — including DMARCbis (RFC 9989–9991), now the official standard as of 2026.

SPF DKIM DMARC ARC BIMI DMARCbis

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07

Content-Type in Detail — How Email Knows What It Contains

Content-Type is the header that tells a mail client whether it's reading plain text, HTML, or an attachment. Here's exactly how that decision gets made.

Content-Type multipart/alternative charset

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08

MIME in Detail — Encoding, Boundaries, and Structure

How MIME boundaries separate message parts, when base64 vs. quoted-printable encoding applies, and how a real multipart email is assembled.

MIME boundary base64 quoted-printable

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09

X-Headers and ESP Headers — The Non-Standard Extensions

X-Mailer, X-Campaign-ID, X-Originating-IP — what X-headers are, why no standard governs them, and how to read the ones your ESP or gateway adds.

X-Mailer X-Originating-IP ESP headers

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10

Spam Scoring Headers — How Filters Make Their Decisions

X-Spam-Status, X-Spam-Score, and Microsoft's SCL/BCL/PCL headers decoded — what triggers a high score, and how to read the rule breakdown filters attach.

X-Spam-Status SCL / BCL / PCL SpamAssassin

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11

Reading and Troubleshooting Headers — From Symptom to Solution

Delayed, bounced, or landing in spam? Learn to read Received headers, Authentication-Results, and timestamps yourself — organized by symptom, not by field.

Troubleshooting Authentication-Results Received chain

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12

Email Forensics — Investigating Through Headers

A structured forensic workflow for suspicious email — preserve, triage, trace Received headers, cross-check authentication, and document findings correctly.

Forensics BEC Spoofing detection

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13

Email Encryption — TLS, S/MIME, PGP, MTA-STS, and DANE

Transport encryption vs. end-to-end encryption in email — how STARTTLS, MTA-STS, and DANE protect the connection, and S/MIME/PGP protect the message.

STARTTLS MTA-STS DANE S/MIME PGP

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14

Inbound Mail Gateways — Routing Modules and the Filtering Pipeline

What actually happens inside an inbound mail gateway — the routing and filtering modules a message passes through before it reaches or misses the inbox.

Filtering pipeline DNSBL Greylisting

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15

SMTP Response Codes — The Complete 4xx and 5xx Reference

Every SMTP response code that matters — 4xx temporary failures, 5xx permanent failures, and the enhanced X.Y.Z codes that explain exactly what went wrong.

4xx / 5xx RFC 3463 Bounce codes

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