substack.com
The three core checks pass. What is missing is the transport and trust layer around them.
4 of 8 checks pass · 4 tools visible in DNS · nothing urgent
1 · Authentication and transport
SPF
PassValid, soft failAnything not on the list is accepted but marked. Google recommends ~all; with DMARC enforced it is as strong as -all.
v=spf1 include:_spf.google.com include:mailgun.org include:mail.zendesk.com ~all
DKIM
Pass2 keys foundSelectors google, cf2024-1 publish a public key.
google._domainkey, cf2024-1._domainkey
DMARC
PassEnforced (p=reject)Failing mail is rejected. This is the finish line.
v=DMARC1; p=reject; pct=100; rua=mailto:re+kvnosxqvppz@dmarc.postmarkapp.com; sp=reject; aspf=r;
MX
Pass2 hostsMore than one receiving server, so a single outage does not bounce mail.
10 mxa.mailgun.org 20 mxb.mailgun.org
MTA-STS
ReviewNot publishedSending servers cannot tell that this domain requires TLS, so inbound mail can be downgraded to plain text by anyone on the path.
TLS-RPT
OptionalNot publishedNobody is told when TLS to this domain fails. Cheap to add once MTA-STS is in place.
BIMI
OptionalNot publishedDMARC is enforced, so this domain qualifies. Yahoo and Fastmail show the logo from the record alone; Gmail and Apple Mail also need a paid VMC or CMC certificate.
DNSSEC
OptionalNot signedOptional. Signing the zone protects every record in this report from tampering.
2 · Tools visible in DNS
| Layer | Tool | Evidence |
|---|---|---|
| Mailbox | Google Workspace | SPF include:_spf.google.com |
| Transactional | Mailgun | SPF include:mailgun.org |
| Support | Zendesk | SPF include:mail.zendesk.com |
| DMARC reporting | Postmark DMARC Digests | DMARC rua=mailto:…@dmarc.postmarkapp.com |
Only tools that leave a DNS trace appear. A shared DKIM selector is attributed only when its CNAME target names the vendor.
3 · Sender requirements (Google, Yahoo, Microsoft)
| Requirement | Applies to | Status | Note |
|---|---|---|---|
| Authenticate with SPF or DKIM | GoogleYahooMicrosoft | Verified | At least one method is in place. |
| Bulk senders: both SPF and DKIM | GoogleYahoo | Verified | Both are present. |
| Publish a DMARC record (p=none is enough) | GoogleYahooMicrosoft | Verified | Policy is p=reject. |
| From: domain aligns with SPF or DKIM | GoogleYahooMicrosoft | Not checkable here | Needs a real message. The Send-a-Test report will check this. |
| Valid forward and reverse DNS for sending IPs | GoogleYahoo | Not checkable here | Depends on the sending server, not the domain. |
| Encrypt in transit (TLS) | GoogleYahoo | Not checkable here | Outbound TLS depends on your sending tool; MTA-STS would cover inbound. |
| One-click unsubscribe (RFC 8058) for marketing mail | GoogleYahoo | Not checkable here | A message header, not a DNS record. Microsoft recommends it rather than requiring it. |
| Spam complaint rate under 0.1% (0.3% is the hard limit) | GoogleYahoo | Not checkable here | Read it in Google Postmaster Tools and Yahoo Sender Hub. Google’s target is 0.1%; 0.3% is where enforcement starts. |
4 · What to fix, in order
-
P3
Publish MTA-STS (record and policy file)
Tells sending servers to require TLS to your MX hosts, which prevents downgrade attacks. The TXT record alone does nothing: the policy file must be served over HTTPS at mta-sts.substack.com. Start in testing mode, watch TLS-RPT, then switch to enforce.
Publish at_mta-sts.substack.com and https://mta-sts.substack.com/.well-known/mta-sts.txtTXT v=STSv1; id=2026100401 mta-sts.txt: version: STSv1 mode: testing mx: mxa.mailgun.org mx: mxb.mailgun.org max_age: 86400
-
P3
Publish TLS-RPT
You get a report when TLS delivery to your domain fails; this is what notices a downgrade that MTA-STS is meant to prevent. The address must exist.
Publish at_smtp._tls.substack.comv=TLSRPTv1; rua=mailto:tls-rpt@substack.com
-
P3
Add a BIMI logo record (optional)
DMARC already qualifies. The logo must be SVG Tiny PS, square, served over HTTPS. Yahoo and Fastmail show it from the record alone; Gmail and Apple Mail also need a paid VMC or CMC certificate, and a VMC needs a registered trademark. Skip this unless the certificate is worth it to you.
Publish atdefault._bimi.substack.comv=BIMI1; l=https://substack.com/.well-known/bimi.svg;
-
P3
Sign the zone with DNSSEC
Protects every record in this report from being swapped in transit. Two steps: enable signing at your DNS host (one switch at Cloudflare and most others), then add the DS record it gives you at your registrar.
Publish atsubstack.com
Appendix · Raw DNS answers
| Name | Type | Answer |
|---|---|---|
| substack.com | MX | 10 mxa.mailgun.org 20 mxb.mailgun.org |
| substack.com | A | 104.18.37.200 172.64.150.56 |
| substack.com | TXT | v=spf1 include:_spf.google.com include:mailgun.org include:mail.zendesk.com ~all (+ 8 other TXT records: site-verification tokens and the like, omitted) |
| _dmarc.substack.com | TXT | v=DMARC1; p=reject; pct=100; rua=mailto:re+kvnosxqvppz@dmarc.postmarkapp.com; sp=reject; aspf=r; |
| <selector>._domainkey.substack.com | TXT | google → key published cf2024-1 → key published |
| _mta-sts.substack.com | TXT | no record |
| _smtp._tls.substack.com | TXT | no record |
| default._bimi.substack.com | TXT | no record |