SocketLabs account administrators can request custom server configuration options, such as requiring the use of a TLS encrypted channel for the delivery of all email messages sent by a specific server, or to a specific domain or mail exchange.
When a requirement is established to use TLS for delivery of a particular message, but we are unable to properly establish a secure connection the following error will be logged:
450 4.7.0 A delivery rule requires the use of TLS for this domain or MX, but we were not able to successfully negotiate an encrypted channel.
When this error occurs, the message will remain in the delivery queue for your SocketLabs server. Message delivery will be retried at least once per hour for up to four days, or based on any customized delivery retry settings established for your account.
If the maximum message delivery retry period has been exhausted and the message still can not be delivered, this error will be logged in the failed messages report for your SocketLabs server.