How to get list of all shared mailboxes within given OU to which given user has Send-As extended rights using powershell without using Exchange Management Shell commands.

The reason why the EMS commands cannot be used : appropriate RBAC role cannot be assigned, which means EMS commands are not accessible.

The background is following: AD forest has dozens of OUs where users and shared mailboxes were created. There are hundreds of users and dozens of shared mailboxes in each OU. The task is to find out what mailboxes given user can send from. The only permissions used is extended right Send-As and never GrantSendOnBehalf.

The search should always be performed against one OU at a time as in "From which shared mailboxes within given OU given user has Send As permission".

Thank you.