Just a quick blog on how you can allow a Distribution Group to receive mail from External Users in Exchange 2010/2007

1. Open the Exchange Management Console

2. Expand Recipient Configuration > Distribution Group

3. Right Click Distribution Group you need and choose properties

4. Go to the Mail Flow Settings Tab > Select Message Delivery Restrictions and choose properties

5. Un-tick “Require that all senders are authenticated”

6. Press ok

You will now receive mail from External users to your Distribution Group