NDR report to sender !!!

Are you talking about somebody, sending you a Email as a Central Email customer or some of the protected user behind Central Email is trying to send a Email out, which does not exists? 

Actually, both users exist, on Sophos Email GW and mail server, but Sophos Email GW gets no existing user from email server (we are investigating reason) for some emails (not all emails) and then deletes email without sending NDR to sender.

Actually, both users exist, on Sophos Email GW and mail server, but Sophos Email GW gets no existing user from email server (we are investigating reason) for some emails (not all emails) and then deletes email without sending NDR to sender.

Central will not generate a email back to the Sender. That would cause to much Email traffic. Be careful on this system, that your Email server has the same kind of data as your Central Email instance. 

Actually, Central will not generate any kind of notification, nor NDR nor some kind of notification to Admins. Everything was setup correctly but due issue with compatibility between Central Email GW and Email server a lot of emails are lost since Central will just DELETE legitimate emails. I have found this very dangerous with Central and we will be moving to some other vendors with better control over this email flow. 

Sophos is afraid of traffic since it is using AWS as provider. Maybe if they have their own servers like some other vendors, maybe things would be different. 

Thanks for your feedback.