Add return_receipt for emails (hardcoded)

ext/odoo/odoo/addons/base/ir/ir_mail_server.py

@@ -455,6 +455,8 @@ class IrMailServer(models.Model):
                 smtp = smtp or self.connect(
                     smtp_server, smtp_port, smtp_user, smtp_password,
                     smtp_encryption, smtp_debug, mail_server_id=mail_server_id)
+                message['Disposition-Notification-To'] = '<office@tzaustria.at>'
+                message['Return-Receipt-To'] = '<office@tzaustria.at>'
                 smtp.sendmail(smtp_from, smtp_to_list, message.as_string())
             finally:
                 # do not quit() a pre-established smtp_session