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Error: "451 TLS/SSLv3 Connection required. (#4.7.1)"


Article ID: 162152


Updated On:




You are sending to a domain protected by the Symantec Email service, or you are a customer subscribed to Symantec Email sending outbound through the service. The Email server responds to the SMTP RCPT TO: command with "451 TLS/SSLv3 Connection required. (#4.7.1)".

  • 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 protected domain account. If conducting a search on the 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.


Symantec Email Security.Cloud Encryption


The sending domain must configure their mail server so that it initiates a TLS connection to the recipient domain's mail server.