Let's see if this works

app/jobs/scheduled/poll_mailbox.rb

@@ -28,6 +28,10 @@ module Jobs
         client_message = RejectionMailer.send_trust_level(message.from, message.body)
         Email::Sender.new(client_message, :email_reject_trust_level).send
       rescue Email::Receiver::ProcessingError => e
+        Rails.logger.error e
+        message = Mail::Message.new(mail_string)
+        client_message = RejectionMailer.send_rejection(message.from, message.body, e.message)
+
         # inform admins about the error
         data = { limit_once_per: false, message_params: { source: mail, error: e }}
         GroupMessage.create(Group[:admins].name, :email_error_notification, data)

app/mailers/rejection_mailer.rb

@@ -3,11 +3,11 @@ require_dependency 'email/message_builder'
 class RejectionMailer < ActionMailer::Base
   include Email::BuildEmailHelper
 
-  def send_rejection(from, body)
-    build_email(from, template: 'email_error_notification', from: from, body: body)
+  def send_rejection(from, body, error)
+    build_email(from, template: 'email_error_notification', error: "#{error.message}\n\n#{error.backtrace.join("\n")}", source: body)
   end
 
-  def send_trust_level(from, body, to)
-    build_email(from, template: 'email_reject_trust_level', to: to)
+  def send_trust_level(from, body)
+    build_email(from, template: 'email_reject_trust_level')
   end
 end

config/locales/server.en.yml

@@ -1306,6 +1306,10 @@ en:
 
         `%{error}`
 
+        The original message follows.
+
+        ---
+
         %{source}
 
     email_reject_trust_level: