After creating a new distribution group in Exchange  and trying to email it from the outside you receive the following error:

Delivery has failed to these recipients or distribution lists:
group@yourorg.com
Your message wasn't delivered because of security policies. Microsoft
Exchange will not try to redeliver this message for you. Please
provide the following diagnostic text to your system administrator.

________________________________
Sent by Microsoft Exchange Server 2007
Diagnostic information for administrators:
Generating server: server.yourorg.com
group@yourorg.com
#550 5.7.1 RESOLVER.RST.AuthRequired; authentication required ##

The reason for this is simple: By default groups are created to no accept anonymous mail, i.e. only authenticated email. This stops your groups being spammed and is great to have on by default, and annoying to have on when you don't know where to look when you want to Internet email enable a group.

In the Exchange console, open the group properties, navigate to the Mail Flow tab and look at the properties for Message Delivery Restrictions.