Received-SPF: None (servername: sender@xs4all.nl does not designate permitted sender hosts)
Exchange returns #550 5.7.1 RESOLVER.RST.AuthRequired; authentication required ##
If you see the error the first thing you want to check is your antispam logs, but the problem can be found somewhere else… In your Exchange server itself!! 
Fix it using the Exchange Management Console:
- Click the Distribution Group name as you want to set
- Click the “Properties“
- Click the “Mail Flow Settings” Tab
- Click the “Message Delivery Restrictions” item
- Click the “Properties…” button
- Uncheck the “Require that all senders are authenticated” check-box
- Click “OK” button
- Click “OK” button again