Imagine you have the following setup: user Y has delegated calendar access (including "Delegate receives copies of meeting-related messages sent to me") to user X, but then user X leaves the company. And then users (internal and external) notice that when they invite user Y to a meeting, they get an NDR of the following type:
Your message did not reach some or all of the intended recipients.
Subject: subject
Sent: 2/12/2008 11:32
The following recipient(s) cannot be reached:
User X on 2/12/2008 11:32
The e-mail account does not exist at the organization this message was sent to. Check the e-mail address, or contact the recipient directly to find out the correct address.The MTS-ID of the original message is:c=US;a= ;p=place;l=EU01S252-081202103204Z-35076
Reason is that user Y will forward the meeting request to user X, who doesn't exist anymore in the organization. There are various ways to resolve this issue, using Outlook, or tools like MDBVue.
But one of the easiest methods is to log into user Y's mailbox using Outlook, select the properties of his calendar, select Property Page "Permissions", and remove there the user object (you won't see the name of User X, but you should see his ObjectSid) listed with a permission level of editor.