How to identify TLS or MTA-STS blocking?

TLS and MTA-STS issue identification:

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.