It is necessary to have mailbox rights delegated to the object (eg. user) that has been deleted and tombstone period has expired, when try to delete this entry using ARS MMC or WI the entry will remain untouched. The following error may be also displayed:
An error occurred during this operation. For details, see the error description below.Administrative Policy returned an error. Failed to make changes to attribute of object. Attribute: edsaMailboxSecurityDescriptor; Object: CN=<mailbox>,OU=Mailbox,DC=<domain>,DC=com.ilbox,DC=
This is a product defect (TF00430254).
This will be fixed on a future release of the product. If you require this immediately corrected, please contact support for a hotfix referencing the defect ID TF00430254.