As the reseller, you can set exchange mailbox permissions on behalf of your customer. Please review the steps below to add or change exchange mailbox permissions using the partner control panel.

1. Navigating to your Control Panel URL or and sign in using your administrative credentials.

2. Once logged in, Select Customer List / Editor, beneath My Customers, to show your available customers.

3. Highlight the customer the user assigned permissions needed to be added to, and click Login as Customer at the top of the Customer List.

4. Now that you are logged in as the customer, navigate to the User List under My users. 

5. From the User List, choose the mailbox you need to assign another user access rights to and click on Edit User.

5. Under the Microsoft Exchange service tab, select Permission Management. From here you will need to Load the Address Book for a list of available Exchange users. You will then have the ability to add/remove user access and set Full Access or Send As permissions. Once completed, hit the Save Changes button to save your settings.