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)".
Symantec Email Security.Cloud Encryption
451 TLS/SSLv3 Connection required. (#4.7.1)
Within the ClientNet portal, the Email Track and Trace tool shows "Not Delivered" in the Delivered column and "Boundary Encryption" in the "Service" column.
A TLS enforcement is configured on the Symantec Email Security.Cloud protected domain account. On the ClientNet portal section Services> Encryption, searching for the 3rd party domain on the TLS Partner tab results in an entry found. This indicates a TLS connection must be used when sending emails to and from the domains in question.
The email administrator of the sending domain must configure their mail server so that it initiates a TLS connection to the recipient domain's mail server.
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