Delayed Message: Stronger security or encryption needed
The email message has not yet been delivered because the message was not sent in an encrypted format, when encryption was expected.
In the sender's email program's account settings, look for options about encryption of outgoing messages. Use options compatible with the sender's email provider.
If this error persists, contact the email provider for help.
4.7.10: Official Definition
A persistent transient failure is one in which the message as sent is valid, but persistence of some temporary condition has caused abandonment or delay of attempts to send the message. If this code accompanies a delivery failure report, sending in the future may be successful.
This indicates that external strong privacy layer is needed in order to use the requested authentication mechanism. This is primarily intended for use with clear text authentication mechanisms. A client which receives this may activate a security layer such as TLS prior to authenticating, or attempt to use a stronger mechanism.