To change mail flow settings for a distribution list, login to the MSPControl and click on Distribution Lists,
or go to Organization > Exchange > Distribution Lists.
You will see all Distribution Lists. 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 list you were looking for, click on its display name.
Go to Mail Flow Settings tab.
Now, you can decide who will be able to send messages to this Distribution List. If you opt for Only senders in the following list, you should specify them in a corresponding field and click Add.
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.