RFCs

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

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.

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

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.

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.

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.

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.

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.