Question : Non-deliverable notifications by Outside of email system

I will in a company that uses exchange and outlook 2007 as a client.  When a user  is no longer with the company, I will get an undeliverable message.

There are contacts outside of the email system that do not receive back non-deliverable notifications.  

Why does this happen.

Answer : Non-deliverable notifications by Outside of email system

As a common practice many organizations turn off sending NDR's on the exchange server. This helps to eliminate possible email loops and also phishing for valid email addresses.