Check and change mailbox permissions using Exchange Management Shell

**List all permissions to a cpecific mailbox
Get-MailboxPermission -Identity "username" | Select User, AccessRights

**Grant full access to an exchange mailbox
Add-MailboxPermission "mailbox_name" -User "user_ID" -AccessRights FullAccess

**Remove full access to an exchange mailbox
Remove-MailboxPermission "mailbox_name" -User "user_ID" -AccessRights FullAccess