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Authentication history · 59 days observed

substack.com

Observed from 4 Aug 2026 to 3 Oct 2026. Nothing published in DNS has moved in that window.

Where it stands today

Live lookup, 3 Oct 2026. The same check /check/substack.com runs.

  • 3fine
  • 1context

  • Looks fine

    present, ending ~all

    Soft fail. Accepted everywhere, though -all is stronger once your sender list is complete.

    v=spf1 include:_spf.google.com include:mailgun.org include:mail.zendesk.com ~all

    See what this looks like →

  • Looks fine

    present with p=reject

    A policy that actually instructs receivers, which is more than most senders publish.

    v=DMARC1; p=reject; pct=100; rua=mailto:re+kvnosxqvppz@dmarc.postmarkapp.com; sp=reject; aspf=r;

    From DMARC p=none is monitoring, not enforcementSee what this looks like →

  • Looks fine

    keys published on 1 selector

    A key existing is not the same as working. Read a real received header and check the d= value matches your before you call this done.

    google._domainkey (Google Workspace)

    From DKIM passing is not DKIM alignedSee what this looks like →

    Part platform, part you

    The key is your platform's to publish and it has. Whether it signs the domain in your is yours to confirm, and cannot show it — send one campaign to yourself and look for =pass header.d=substack.com in the Authentication-Results header.

  • Context

    MX records present

    Where you receive mail says nothing about where you send it. Marketing sends usually leave through a different platform entirely.

    mxa.mailgun.org, mxb.mailgun.org

    See what this looks like →

What has moved

One entry per day a published record actually changed. Days we looked and found nothing different are counted, not listed.

  1. First observation — what was already published

    SPF published.

    v=spf1 include:_spf.google.com include:mailgun.org include:mail.zendesk.com ~all

    DMARC published.

    v=DMARC1; p=reject; pct=100; rua=mailto:re+kvnosxqvppz@dmarc.postmarkapp.com; sp=reject; aspf=r;

    DKIM keys on selectors we probe.

    google._domainkey (Google Workspace)

    MX records present.

    mxa.mailgun.org, mxb.mailgun.org
Where this comes from. Public DNS, and nothing else. We read the same TXT and MX records any mail server reads before accepting a message, on the days someone looked. There is no scan, no login, no mail, and no score here — only what was published and the date we saw it. Gaps are days we did not get a clean answer from a resolver, and we would rather leave those blank than guess at them.
Run the full checkSee which rules are yours