Some of the "official" causes for that sort of error are:
Microsoft Dynamics CRM relies on the local SMTP server to forward email messages to the Exchange server. This problem occurs for one of the following reasons:
The SMTP configuration on the Microsoft Dynamics CRM server is not configured to forward email messages to the Exchange server.
The Exchange server is not configured to allow relay messages from the Microsoft Dynamics CRM server.
Do you have receive connectors configured on your Hub Transport?
Since your ussing Exchange 2007, its reccomended that you create a receive connector , then to config it for anonymous use, setup protocol permissions for the receive connector. The restart the transport service.
You are going to have to use Adsiedit to setup the permissions.
Once in there, navigate through the config to Configuration|Services|Exchange| the CN|Administrative Groups|the Exchange admin group|servers|Protocols and then to SMTP receive connectors.
Go to the properties and the security tab. Select the anonymous logon option. Then select
"Submit messages to any Receipint and Accept authoritive domain sender"
Then let's restart the service.
Let us know if it helps.
Curtis J Spanburgh