Delayed Message: Security issue with user authentication
The email message has not yet been 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 system does not understand.
Fixes
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.
4.7.9: Official Definition
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that start with 4.X.X:
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.
[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.