You need this script, even if you don't know you need it. :-)
If you manage a group of email users, over time you have probably granted permissions for various users over other user's mailboxes.
This is really common. For example, an administrative assistant will often need access to open her supervisor's mailbox. And also to send mail as if she was the supervisor.
Another example is when a support representative will need to send email as if the representative was the destination support mailbox.
These represent three common special permissions: Send-As, Send-On-Behalf, and Full-Control.
This script reports on a select set of mailboxes, and who has these special permissions on those mailboxes.
With the Filter and OrganizationalUnit (OU) parameters, the administrator executing the script can easily restrict the inspected mailboxes to a desired subset.
This is common information you need to properly audit access to mailboxes in an on-premises Exchange environment - or in Office 365. This script works with both on-premises Exchange mailboxes and Office 365 mailboxes.
When you have decided to migrate to Office 365, you need to know what permissions are assigned to which mailboxes, in order to duplicate these permissions in the cloud environment.