To change mail flow settings for a contact, login to the MSPControl and click on Contacts,
or go to Organizations > Exchange > Contacts.
You will see the list of all existing contacts. If you have many, it might be a good idea to use Search form in the right to find the one you need. You can choose whether you want to search by Display Name or E-mail Address.
Once you found the contact you were looking for, click at them.
You will see two tabs, General and Mail Flow Settings. Click the second one.
Now, you can decide who will be able to message this contact. If you opt for Only senders in the following list, you should specify them in a corresponding field.
If you thick Require that all senders are authenticated, any messages from senders that don't have valid logon credentials in your organization will be rejected.
You can also choose if you want messages from certain addresses to be rejected by choosing the corresponding option.
When you're done here, click either Save Changes or Save Changes and Exit. The first option will leave you at this page, the second will redirect you to the list of all contacts.