You are sending to a domain protected by the Symantec Email Security.cloud service, or you are a customer subscribed to Symantec Email Security.cloud sending outbound through the service. The Email Security.cloud server responds to the SMTP RCPT TO: command with "451 TLS/SSLv3 Connection required. (#4.7.1)".
A TLS enforcement is configured on the Symantec Email Security.cloud protected domain account. If conducting a search on the Symantec.cloud portal section Dashboard > Services > Email Services > Encryption, for the 3rd party domain on the TLS Business Partners tab results in an entry found, it indicates a TLS connection must be used when sending emails to and from the domains in question.
The sending domain must configure their mail server so that it initiates a TLS connection to the recipient domain's mail server.