Manager sends the meeting request and the attendees accept it and immediately they get and NDR stating "The following recipient(s) could not be reached" with a deleted user account's name in it.
This can happen if the deleted user had been delegated access to the managers mailbox and the check box which states "Send meeting requests and responses only to my delegates not to me" had been checked in outlook delegates tab.
In certain cases you would still see the check box ticked and grayed out even after the delegatee has been removed from the outlook setting. You could add someone else as a delegatee and then remove the tick and then remove the delegatee again to clear the checkbox.