Hooray! IT works!
self.add_model_callback(UserEmail, :after_commit, on: :update) do
puts "#{'-'*50}\nEMAIL YES ADDRESS IS UPDATED for #{self.user_id}!!!!\n#{'-'*50}\n"
user = User.find(self.user_id)
GroupDomain.add_to_group_if_in_whitelisted_domain(user)
end
Thanks a million, @Falco!