Meeting organizers are receiving NDR’s

Question:
A conference room mailbox is forwarding meeting requests to many recipients. Some users have already left the company and don’t have a mailbox anymore. Now, meeting organizers are receiving NDR’s from these mail addresses.
I searched the conf room mailbox properties in AD, and in Outlook, but couldn’t find where to remove these invalid addresses.

Answer:
Those users had probably delegate permissions on the conference room.
You will have to recreate this user (same display name and email address) on your Domain and then you will see them under the delegate permission of the conference room.
Open Outlook -> Tools -> Options -> Delegate. Remove them from there.
Now the meeting organizers should not receive an NDR anymore!