From: DeskNow Knowledge Base
http://www.desknow.com/kb/

Can I setup DeskNow to reject messages immediately if they are blacklisted by SURBL?
http://www.desknow.com/kb/idx/4/129/article/

This is not possible, because of the nature of SURBL.

SURBL blacklists addresses that are referred to in the body of the message. So DeskNow must receive the message entirely before analyzing it and checking with SURBL for blacklisted addresses.