Exchange 2010 Antispam Sender Filtering

I had a issue where a external person couldn't email a client with Exchange 2010.  They got a sender denied NDR.  I disabled the sender filter  on the 2010 server and the external user was then able to email my yclient.  When I reviewed the options for the sender filter, I didn't see any entries/configuration so I was wondering why they were blocked??

Thanks!