XADM: Administrator Able to Change Permissions for Mailbox without Permissions Admin. Right

An Exchange Server administrator with the Admin. role can edit and change mailbox permissions on the Permissions tab when the mailbox is initially created. If the same administrator attempts to modify or change the same attributes on an existing mailbox at a later time, he or she receives an error stating that he or she has insufficient access rights.

While creating the mailbox, the assignment of permissions and other operations are treated as a single task. Because users with the Admin. role can create mailboxes, they can assign permissions while creating mailboxes.

This behavior is by design.