I used to have some clients that weren’t willing to spend a monthly fee on antispam and to be fair, they didn’t get heaps. I impemented Exchange 2010’s antispam feature and assigned some main blacklists. A client of theirs (funnily enough at the time an ex client of mine (I changed jobs)) was blacklisted, but they urgently needed emails from them.
Exchange 2010 doesn’t have a basic feature to simply whitelist, so the following guide came to my rescue.
Whitelisting email addresses in Exchange 2010 anti-spam | chipperfields.net.