Exchange Server 2007 includes several Anti-Spam Message Filtering features as standard.
Within Small Business Server 2008, these are all enabled by default. In my experience, this can result in important incoming messages failing to arrive at the intended recipients inbox. To make matters worse, neither the System Administrator or the User seems to receive any notification that messages are being blocked.
If you have evidence to suggest that important emails are “missing in transit”, I recommend you review these “Anti-Spam” settings as soon as possible.
Open Exchange Management Console and navigate to: "Organization Configuration => Hub Transport => Anti-Spam", you will see the various options available to you. The "Sender Reputation" feature is most likely causing the trouble. I recommend turning the "Sender Reputation Block Level Threshold" up to "9" and setting the “Threshold Action” to Zero. Alternatively, disable the “Sender Reputation” feature completely.
In addition, take a look within "Server Configuration => Hub Transport => Anti-Spam => IP Block List" and make sure the "Sender Reputation" feature (above) hasn’t automatically populated this list with IP Addresses it decided were not to be trusted. You will not be able to receive emails from any servers that are in this list.