When you disable an account it does not automatically remove the name from distribution lists. One would think somehow this would have been included in the process of disabling and deleting accounts, but it is not. You have to remove the user from the distribution lists manually either in AD after the account is disabled or deleted, or else have the presence of mind to go to the “members of” tab before deletion and remove them from the groups there first. Also be aware that if anyone has them as a delegate users will also get a non delivery report when they send a meeting request to someone who has the deleted account in the delegates list. This is why our company discourages use of delegates and ask users to give people permissions to their calendar folder instead.