We recently ran in to this problem, and it took us a bit of time to track down.
We created a mail enabled group in exchange 2007, which worked great internally, but would reject messages sent from external senders with a "550 5.1.1 User unknown" error.
The fix? go to the group properties, "mail flow settings" tab, "message delivery restrictions"
un-check the box "require that all senders are authenticated"
Really, it should be obvious, but it took us a while to figure out, and certainly any exchange admin should know it...