Exchange 2007 introduces a built-in feature called Sender Reputation for both the Edge and Hub Transport server roles. The purpose of Sender Reputation is to record the legitimacy, through a number of tests, of each external SMTP server that sends email to Exchange. For detailed information on how Sender Reputation works, please visit the following link: http://technet.microsoft.com/en-us/library/bb124512.aspx
By default, SBS 2008 is aggressive in blocking suspicious senders, and since all inbound e-mail is coming from the same sending server, there is a risk that the hosting company server could be incorrectly blocked. This feature will eventually block an offending host for 24 hours.