RFCs

• ESMTP Extensions for Senders — SIZE, PIPELINING, CHUNKING, DSN

  What RFC 1870 SIZE, RFC 2920 PIPELINING, RFC 3030 CHUNKING/BINARYMIME, and RFC 3461 DSN (NOTIFY/RET/ENVID/ORCPT) do, and when each matters on an ESP's outbound path.

• MIME & Transfer Encodings (RFC 2045/2046)

  Deliverability-focused MIME digest: structure and headers, multipart/alternative ordering, quoted-printable and base64 rules and pitfalls, size inflation, and the malformed-MIME defects that trigger filtering.

• RFC 3834 — Automatic Email Responses

  The auto-responder rulebook: when not to respond, the Auto-Submitted header, null return-path and loop prevention, and how senders should emit and handle OOO/vacation replies.

• RFC 5321 — SMTP: The Transport Protocol

  Deliverability-focused digest of SMTP: session model, envelope vs. content, reply-code semantics, null sender, retry rules, size limits, and timeouts.

• RFC 5322 — Internet Message Format

  Deliverability-focused digest of the message format: required vs. optional headers, From/Date/Message-ID/Reply-To rules, line limits, and malformations that trigger filtering.

• RFC 5598 — Internet Mail Architecture

  The vocabulary of email: MUA/MSA/MTA/MDA, ADMDs, actor roles, mediators vs. relays, and which actor sets which identity field.

• RFC 6647 — Email Greylisting

  How greylisting works (tuple mechanics, timing windows, reply codes), the IETF recommendations for greylisters, and what senders must do — same-IP retries, pool interactions — to pass it.

• SMTPUTF8 / Internationalized Email (RFC 6530/6531/6532)

  EAI digest: the SMTPUTF8 SMTP extension, UTF-8 headers, the no-downgrade rule, error codes, and what an ESP must decide about accepting and delivering internationalized addresses.