Email-ext sometimes show warning that a user does not have permission or is not a known user :

Not sending mail to user <userEmail> with no permission to view <jobFullName> 

However this can be misleading if for example there is inconsistency between SCM author user name and Jenkins authenticated user ID. Or more generally, if several users in Jenkins have the same email address configured. How do you know which one we are talking about here ?

Those message could maybe show the user ID to help users check about what is described by it.