MTA-STS Policy Checker
Check MTA-STS TXT records and mta-sts.txt policy files for RFC 8461 syntax, MX hosts, mode, and max_age.
Validate MTA-STS DNS TXT and policy files
MTA-STS (Mail Transfer Agent Strict Transport Security), defined in
RFC 8461,
lets a domain publish which MX hosts are authorized to receive mail over TLS. Senders fetch a DNS TXT record at
_mta-sts.example.com and a policy file at
https://mta-sts.example.com/.well-known/mta-sts.txt before upgrading SMTP connections.
This checker validates both pieces locally: the TXT record must include v=STSv1 and an
id token, while the policy file must declare version: STSv1, a supported
mode (enforce, testing, or none), at least one
mx: host pattern, and max_age of at least 86,400 seconds (one day).
When to use this tool
- Before publishing a new MTA-STS policy after MX or TLS changes.
- When auditing a vendor export of DNS TXT and HTTPS policy content.
- While switching from
testingtoenforcemode.
Related: DMARC Record Builder, DKIM Record Decoder, and DNS Zone File Validator.
Frequently Asked Questions
Does this tool fetch my live DNS or HTTPS endpoints?
No. Paste the TXT value and policy file body; validation runs entirely in your browser.
What does the policy id do?
The id in DNS must change whenever you update the HTTPS policy so senders know to refresh the file.
Are wildcard MX patterns allowed?
Yes. RFC 8461 permits patterns such as *.example.com on mx lines.
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