Start here: fix a failing DMARC check
DMARC is the protocol that ties SPF and DKIM together — and it's the one most senders get wrong. If your aggregate reports show alignment failures, or messages are being silently quarantined by major receivers, fix this before anything else.
For a focused walkthrough of SPF-to-From alignment, read How to Fix DMARC Failing.
All DNS & authentication guides
Manually query the envelope domain’s published SPF TXT policy and read the receiver’s authentication result.
Read guide →Trace duplicate policies, missing or incorrect includes, lookup-limit PermErrors, message results, and DMARC alignment.
Read guide →Add an SPF record to your DNS in four steps — list every sending source, build the include chain, publish the TXT at your apex, and validate it.
Read guide →Step-by-step PTR setup — why reverse DNS matters for deliverability, who controls the record, and how to verify it resolves with forward-confirmed reverse DNS.
Read guide →Set up MX records for inbound mail delivery — priority weighting, multiple records for redundancy, MX lookup verification, and the SPF/DKIM/DMARC tie-back.
Read guide →A complete overview of the three email authentication protocols — what each one does and how they work together as one system.
Read guide →Match a real message’s DKIM selector and signing domain to its published DNS key, check the sending provider, and distinguish DKIM pass from DMARC alignment.
Read guide →Why your DMARC check fails, SPF vs DKIM alignment, the seven most common failure causes, and a step-by-step fix.
Read guide →The 2024 Gmail bulk sender checklist — SPF, DKIM, DMARC, spam rate, one-click unsubscribe, and From: alignment.
Read guide →Verify your domain in Google Postmaster Tools, read domain and IP reputation scores, troubleshoot a "Low" reputation, and tie it back to your authentication stack.
Read guide →Enroll in Microsoft SNDS and JMRP, read your Outlook.com IP reputation and complaint data, and connect it back to your SPF/DKIM/DMARC posture.
Read guide →Read 550 5.7.X authentication-class bounces from Gmail and Microsoft, and every other common 4xx and 5xx response — hard vs soft bounce, what to retry, what is an SPF/DKIM failure signal.
Read guide →A broad overview of what SPF, DKIM, and DMARC authenticate, how alignment works, and how to inspect DNS and message results.
Read guide →Also check: are you on a blocklist?
Misaligned DMARC often coexists with blocklistings. A failing authentication posture at high volume gets you flagged by Spamhaus, Cisco Talos, Barracuda, and SORBS faster than anything else.
Run the free blocklist check →