Email Authentication: SPF, DKIM, DMARC

1. SPF check_host

   Evaluating the SPF policy for a sender IP and domain.

2. SPF mechanisms and modifiers

   ip4, include, a, mx, all, redirect and qualifiers.

3. DKIM protocol elements

   DKIM-Signature header, canonicalization and key records in DNS.

4. DKIM signer and verifier actions

   Signing and verification procedures.

5. DMARC policy record

   DMARC TXT record tags: p, rua, pct, adkim, aspf.

6. DMARC identifier alignment

   Relaxed vs strict alignment of From domain with SPF/DKIM.

7. MTA-STS

   Policy for requiring TLS on SMTP delivery.

Each section links to the official page. Titles and summaries by Extra Credit; the docs belong to their authors.