add email events

app/mailers/user_mailer.rb

@@ -6,6 +6,8 @@ class UserMailer < ApplicationMailer
     @user = user
     @token = @user.send(:set_reset_password_token)
 
+    assign_message_metadata('user_invitation', @user)
+
     mail(to: @user.friendly_name,
          subject: 'You are invited to DocuSeal')
   end