Can we restrict sending notifications only to internal employees

i'm just wondering how is everybody out there implementing or even testing in non-production environment the notification process. I dont want the notifications sent out to the customers or vendors but just only to the internal employees and any non-email office should be blocked. One way of testing I did is to have one mail box that all the notifications are sent to but would like to listen what others are doing as well.

But, I would like to figure out a way if we can block all the non-office email addresses.

Thanks