Internet Protocols · Cluster
Email Protocols
Submission, delivery, and authentication.
- 01 Envelope and Header Addresses in SMTP Delivery Why a message carries two independent sets of addresses, which one routing and bounces actually consult, and what breaks when a gateway rewrites one but not the other.
- 02 Header Folding and the Line Limits That Shape MIME Encoding How the 998-octet line limit, dot-stuffing, and the 75-character encoded-word bound force the transfer encodings that mail still uses.
- 03 The Split Between Submission and Relay Ports Why port 587 exists separately from port 25, which policies differ between the two roles, and how a server distinguishes a user from a peer MTA.
- 04 Queue Management and Retry Scheduling in MTAs How a mail server decides when to retry a failed delivery, why the four-hour and five-day thresholds exist, and how a permanent failure becomes a bounce.
- 05 MX Resolution, Preference Ordering, and Null MX How a sending server turns a domain into a list of hosts, why the preference values are ordered the way they are, and what an empty MX list asserts.
- 06 ESMTP Extension Negotiation and Its Failure Modes How EHLO replaced HELO, what the keyword list in the reply actually guarantees, and why PIPELINING and CHUNKING change the shape of a session.
- 07 SPF Evaluation and Its Ten-Lookup Budget How a receiver matches a sending IP against a domain policy written in TXT records, why the recursion limit exists, and what each result value actually permits.
- 08 DKIM Canonicalization and What It Actually Signs How a DKIM signature survives transport rewriting, why two canonicalization algorithms exist, and which header fields the signature deliberately excludes.
- 09 DMARC Alignment Between Header and Envelope Domains How DMARC combines two independent authentication results into one verdict, why alignment is stricter than either check, and what the policy record actually authorizes.
- 10 ARC and Authentication Across Forwarding Hops How a chain of seals lets a receiving domain trust authentication results recorded before a message was forwarded, and where that trust stops being safe.