What does “TLS negotiation failed” mean?

TLS negotiation failed indicates encryption handshake problems:

What it means:

The sending and receiving servers could not establish a secure TLS connection. The cryptographic handshake did not complete successfully.

Common causes:

Certificate problems: Expired, self-signed, or invalid certificates. Protocol mismatch: Incompatible TLS versions. Cipher suite issues: No common encryption algorithms. Configuration errors: Misconfigured TLS settings.

Typical codes:

454 (TLS not available). Various connection errors during STARTTLS.

Response:

Check your TLS configuration. Verify certificates are valid. Some servers may allow fallback to non-TLS. May be receiving server's problem.

TLS failure is a failed secret handshake. The servers could not agree on how to communicate securely.