Exchange 2010 says #550 5.7.1 RESOLVER.RST.AuthRequired

If you make distribution groups in Exchange 2010 and you are enabling them from external senders, make sure you remove the authentication requirement on the group itself. Follow these steps to ensure its setup proper.

#1. Click the Distribution Group name as you want to set

#2. Click the "Properties"

#3. Click the "Mail Flow Settings" Tab

#4. Click the "Message Delivery Restrictions" item

#5. Click the "Properties..." button

#6. Uncheck the "Require that all senders are authenticated" item

#7. Click "OK" button

#8. Click "OK" button again

Thats all you need to do to enabled external senders.