"Delivery to the following recipients failed permanently.... Reason: Permanent Error"

"Delivery to the following recipients failed permanently.... Reason: Permanent Error"

We have 11 users - only one is having a problem.
When I send email to shannon@tworoadswellnessclinic.com I get a return saying >
"Delivery to the following recipients failed permanently.... Reason: Permanent Error"
what do I need to do to correct this?
Thanks