Weird Exchange 2007 errors. I had a system that was sending updates via smtp but not working. I whitelisted the IP address of the server on the edge server under anti-spam, like you do, but the hub transport started denying the message with “550 5.7.1 RESOLVER.RST.AuthRequired; authentication required”. It took me a while to notice that I was getting the 5.7.1 error on the Hub Transport role rather than the Edge role, as I’ve grown used to having to log into the Edge role to deal with any anti-spam stuff (5.7.1 means anti-spam).
Turns out on the Hub, EMC, Recipient Configuration, Distribution Group, Properties of the Group, Mail Flow Settings Tab, Message Delivery Restrictions, Properties there is a check box for “Require that all senders are authenticated”.
That is far, far too many levels deep for a new default setting.