Use receives error message, "Some items cannot be deleted. They were either moved or already deleted, or access was denied." , when trying to delete items from a delegated calendar in the Outlook client.

This was due to the Outlook client being configured to place deleted items in the delegated mailboxes "Deleted Items" folder rather tyhan the users own and not having access to write to that folder.

It can be fixed by adding a folder permission to the deleted items folder as below or the user can use shift+delete to bypass using the trash folder.

Add-MailboxFolderPermission -Identity "$mailbox:\deleted items" -User $username -AccessRights Author