Allow a distribution group to receive external emails in Exchange 2010

What:

Microsoft Exchange 2010 on Windows Server 2008 R2

Problem:

People from outside the company can't email distribution group.

Solution:

1. Log on to your Exchange server
2. Open up "Exchange Management Console"
3. Navigate to "Recipient Configuration > Distribution Group"
4. Locate group on the list, right click on it and select Properties
5. Click on Mail Flow Settings > Message Delivery Restrictions and click Properties
6. Uncheck "Require that all senders are authenticated" and save it