Regularly people suggest something like TMDA or Active Spam Killer as the "solution" to the spam problem - they reply to unknown senders getting them to whitelist themselves. This always seemed incredibly annoying to me and in fact I got bitten by it when someone emailed me a question about one of my modules, so I spent a good 20 minutes composing a reply only to get a TMDA bounceback suggesting I should validate myself. It was just too much - he emailed ME unsollicited and I shouldn't have to jump through hoops to get my email reply to him.
Anyway, I saw this nugget on Slashdot (hey, it happens occasionally!):
I'm amazed by this user-hostile suggestion every time I hear it. Suppose you post your resumé on Monster.com. Who are you going to whitelist? Suppose your friend changes ISPs and then tries to e-mail you his new address? It won't be whitelisted, so it will bounce. Suppose to fill out a tech support request form. You don't know the address of the person that will contact you (or even if they will be the same domain as the web site).
I thought that summed up my thoughts on this quite well.