Message Not Delivered: Stronger security or encryption needed
The message could not be delivered because the message was not sent in an encrypted format, when encryption was expected.
Fixes
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.
5.7.10: Official Definition
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that start with 5.X.X:
A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made for successful delivery.
[RFC5248] (Best current practice) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.7.10:
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.