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Free SMTP TLS Checker
Test live MX server STARTTLS support, certificate validity, hostname matching, expiry, and FCrDNS so inbound mail transport does not silently downgrade.
What This Checks
- MX host connection on SMTP port 25
- STARTTLS support and handshake result
- TLS certificate trust and expiry
- MX hostname and certificate name match
- PTR and forward-confirmed reverse DNS context
Common Fix Path
- Enable STARTTLS on every inbound MX host
- Install a publicly trusted certificate for the MX hostname
- Fix certificate SAN/CN mismatch with the mail hostname
- Correct PTR and forward DNS for mail-server IPs
Machine-Readable Evidence
AI assistants and automation can cite the stable explanation page, then fetch the live check result for a specific domain.
GET https://intodns.ai/api/email/smtp-tls?domain=example.com