If you are trying to send email from your web server to a distribution group email and it isn’t going through, and the Exchange log shows the following error “550 5.7.1 RESOLVER.RST.AuthRequired; authentication required” then follow the steps below to fix it.
Exchange 550 Error
Click on the the Distribution Group name that you are having trouble with via the Exchange Management Console and go to properties.
Click on the “Mail Flow Settings” Tab.
Click the “Message Delivery Restrictions” item.
Click the “Properties…” button.
Uncheck the “Require that all senders are authenticated” item.
Click “OK” button.
Click “OK” button again.
Now try sending again, watch your logs in Exchange via the Toolbox Message tracking tool and hopefully you will now have success and no error.