Distribution Group Doesn't Accept Incoming Mail from Internet, Exchange 2010

In Exchange 2010, the default settings won't allow email inbound from outside senders. This is due to a requirement that the senders be authenticated.

To fix, in the Exchange admin tool, double click on the distribution group. Click the Mail Flow tab, and then double click on Message Delivery Restrictions. Uncheck "Require that all senders are authenticated."

Additionally, I found that creating the distribution group in AD didn't cause it to migrate correctly to Exchange. It showed up, but had no members. So I had to fix that as well. That might be a fluke, or maybe I didn't give AD enough time.

Lastly, the AD distribution groups that were imported when we migrated to this Exchange server all had correct permissions.

So, perhaps making the distribution group in AD is the correct way - but you may have to wait until the group fully migrates. Sometimes, you just can't win. :(