slack.com
Receivers are told to reject mail that fails authentication for this domain.
Receivers are told to reject mail that fails authentication for this domain. Keep watching aggregate reports so a new sending service does not silently start failing.
Score held up: DMARC is enforcing at p=reject, so nobody else can send as this domain. That is the hard part, and it is done. What is left below is about your own mail arriving, not about anyone spoofing you.
156 DNS queries · 2246 ms
- 1Selector 'smtpapi' uses a 1024-bit RSA keyRotate to 2048-bit at your next key rotation.
- 2Selector 'mailo' uses a 1024-bit RSA keyRotate to 2048-bit at your next key rotation.
- 3No explicit subdomain policy (sp=)Consider adding 'sp=reject' if no subdomain sends mail.
- 4No MTA-STS policyPublish a TXT record at _mta-sts.slack.com and a policy file at https://mta-sts.slack.com/.well-known/mta-sts.txt.
Identity ·Sending services detected
Read from your SPF includes and MX hosts, then checked for a published DKIM key.
Identity ·DMARC
- Policy
- reject
- Coverage
- 100%
- Alignment
- dkim=r spf=r
Identity ·DKIM
111 selectors probed
qualtricsQualtricsRSA 2048zendesk1ZendeskRSA 2048zendesk2ZendeskRSA 2048smtpapiSendGridRSA 1024mailoMailgunRSA 1024Identity ·SPF
Worst-case evaluation across every include.
- include:_spf.qualtrics.com
- include:mail.zendesk.com
- include:_spfextra.slack.com
Findings (7)
MediumSelector 'smtpapi' uses a 1024-bit RSA key›
1024-bit is the RFC 8301 floor, not a good target. 2048-bit is the current recommendation and is universally supported.
Rotate to 2048-bit at your next key rotation.
RFC 8301
MediumSelector 'mailo' uses a 1024-bit RSA key›
1024-bit is the RFC 8301 floor, not a good target. 2048-bit is the current recommendation and is universally supported.
Rotate to 2048-bit at your next key rotation.
RFC 8301
LowNo explicit subdomain policy (sp=)›
Subdomains inherit p=, which is usually what you want. Setting 'sp=reject' explicitly protects unused subdomains even while the parent is still at p=none.
Consider adding 'sp=reject' if no subdomain sends mail.
LowNo MTA-STS policy›
Without MTA-STS, mail to your domain can be downgraded to plaintext by an active network attacker, because SMTP's STARTTLS is opportunistic by default.
Publish a TXT record at _mta-sts.slack.com and a policy file at https://mta-sts.slack.com/.well-known/mta-sts.txt.
RFC 8461
LowNo TLS-RPT record›
Sending servers have no address to report TLS failures on mail to your domain, so transport problems stay invisible to you until someone complains.
Publish 'v=TLSRPTv1; rua=mailto:<address>' at _smtp._tls.slack.com.
RFC 8460
InfoEligible for BIMI›
Your DMARC policy is already enforcing, which is the hard prerequisite for BIMI. Publishing a logo is now largely a branding exercise.
Publish an SVG Tiny PS logo plus a VMC, then add a default._bimi record.
InfoEligible for DANE›
Your zone is signed, which is the hard prerequisite for DANE. Publishing TLSA records for your MX hosts would let sending servers pin your certificates without relying on the public CA system.
Publish a TLSA record at _25._tcp.<each MX host> matching your MX certificate.
RFC 7672
Show 15 passing checks
PassAggregate reporting is configured›
Aggregate reports are requested at: [email protected], [email protected].
PassExternal report destination vali.email is correctly authorised›
Authorised by slack.com._report._dmarc.vali.email
PassExternal report destination inbox.ondmarc.com is correctly authorised›
Authorised by slack.com._report._dmarc.inbox.ondmarc.com
PassExternal report destination inbox.ondmarc.com is correctly authorised›
Authorised by slack.com._report._dmarc.inbox.ondmarc.com
PassDMARC policy is p=reject›
Mail failing authentication is rejected outright. This is the correct end state.
PassSPF lookup count is healthy (3/10)›
Worst-case evaluation stays within the limit with headroom to spare.
PassSPF ends in '-all' (hard fail)›
Unauthorised hosts are explicitly rejected. This is the correct end state.
PassQualtrics's DKIM key is published at qualtrics._domainkey›
Detected via SPF include:_spf.qualtrics.com. A valid public key for this service is published at qualtrics._domainkey.slack.com.
PassZendesk's DKIM keys are published at zendesk1._domainkey, zendesk2._domainkey›
Detected via SPF include:mail.zendesk.com. A valid public key for this service is published at zendesk1._domainkey.slack.com, zendesk2._domainkey.slack.com.
Pass5 DKIM key(s) published›
Found at: mailo, qualtrics, smtpapi, zendesk1, zendesk2.
PassSelector 'qualtrics' uses a 2048-bit RSA key›
Key length meets current guidance.
PassSelector 'zendesk1' uses a 2048-bit RSA key›
Key length meets current guidance.
PassSelector 'zendesk2' uses a 2048-bit RSA key›
Key length meets current guidance.
PassZone is signed and validates›
A validating resolver reported this zone's answers as authenticated. Resolvers that validate DNSSEC reject a tampered answer for your SPF, DKIM and DMARC records rather than pass it on. This also makes DANE available to you.
PassCertificate issuance is restricted by CAA›
Only digicert.com, letsencrypt.org may issue certificates for this domain, so a certificate for your mail hosts or your MTA-STS policy endpoint from any other authority would be a mis-issuance.
RFC 8659
Transport and zone
Where your mail lands, and whether the DNS carrying your records can be trusted.
This is the configuration. It is not the whole picture.
A DNS scan cannot tell you whether slack.com’s mail actually aligns, because alignment is decided per message at the receiver. Collect DMARC aggregate reports to see every source sending as this domain.