It would be very comfortable and helpful, if ACLs on Mailboxes were more differentiated. Particulary a differentiation between mailbox owner and those who have additional access to that mailbox is important when it comes to critical operations like deleting a entries.

Example:
If a user (e.g. secretary) has the rights to add and delete entries in a mailbox of his directors or colleague, he can not only delete the contacts and appointments which he added to this mailbox, but also the contacts and appointments which were maintained by the mailbox owner.

So for business use cases it would be a great improvement, if MDaemon would offer a more complex ACL-Model (like the possibilities of Outlook/Exchange).