Message Not Delivered: Security issue with user authentication
The message could not be delivered because the sender's email program attempted to authenticate you to the sender's email program (to prove your identity) using a language that the sender's email provider email system does not understand.
In the sender's email program's account settings, look for "SMTP" (which is about sending email) and, in the SMTP area, "authentication". There should be options for authentication type. Either find out which one the sender's email program recommends, and select that one, or find an option that works by trying each option in turn.
If this error persists, contact the email provider for help.
5.7.9: 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.
[RFC4954] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.7.9:
This response to the AUTH command indicates that the selected authentication mechanism is weaker than server policy permits for that user. The client SHOULD retry with a new authentication mechanism.
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