How to identify TLS or MTA-STS blocking?
TLS problems:
- Certificate expired or invalid
- Protocol version mismatch
- Cipher suite incompatibility
- Connection refused or failed
MTA-STS enforcement:
- Receiving domain requires TLS
- Your server cannot meet requirements
- Mail rejected before content check
Symptoms:
- Sudden delivery failures to specific domains
- TLS handshake errors in logs
- Connection timeouts
Diagnosis:
- Check SMTP logs for TLS errors
- Test TLS connection manually
- Verify certificate validity
- Check receiver's MTA-STS policy
Secure channel requirements not met. Verify your encryption meets port standards.
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