Since Exchange 2007 there is a new behaviour in Microsoft's Exchange Server for distribution groups. If an e-mail comes from an external server (e.g. from internet) the e-mail might be rejected and the sender might never receive an error message. This results in confusion on both sides: Where the hell did the mail go?!
The resolution is simple. The Exchange logs (Message Tracking) show the following error message with an EventId FAIL:
550 5.7.1 RESOLVER.RST.AuthRequired; authentication required
By default, distribution groups are "made for internal purposes" and not for e-mails received by internet. Open the Exchange Management Console and open Recipient Configuration -> Distribution Group. Right-click on your distribution group and select Properties. 
In the 'Mail Flow Settings' tab click on 'Message Delivery Restrictions' and click on Properties.
You will see, that 'Require that all senders are authenticated' is activated. Because emails from internet cannot be authenticated (by Active Directory obviously) they will not be accepted. As soon as you deactivate this option, e-mails from internet or other external sources will be accepted.
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