Hi @saad2,
Welcome back to the Auth0 Community!
This issue occurs on the custom email provider’s end and means that the email provider is blocking the permission of an authenticated user or application to send emails. If you are using something like Sendgrid for your External SMTP Provider you can trying updating the API keys by enabling Full Access to the assigned permissions.
You can check out this article on the matter - Sending Email Fails with Error “Authenticated user is not authorized to send mail” and can look through the API key permissions information from Sengrid side as well.
I hope this helps your case, and if you have further questions please let me know!
Kind regards,
Remus