the c api lets you get at the smtpconnect function and allows you to bypass the dnsbl check but it also bypasses the relay check as well (as if its a single function), and thats not what you'd normally want to happen
i wrote this addin

, it will intercept dnsrbl tagged mail (it's configurable) and moves it to another database, no reason you couldn't put an agent in there, to give a sort of after effect whitelist function that moves any appropriate documents back into the mailbox, you'd still end up receiving the mail which is a waste of bandwidth, but you could always just use this to keep track of what would have been rejected and if you don't get too much collateral damage just change the setting to log and reject