This already happens! I've even received a (slightly) researched, personalised Powerpoint presentation from a company out of the blue. I didn't buy, but intere

This already happens! I've even received a (slightly) researched, personalised Powerpoint presentation from a company out of the blue. I didn't buy, but interestingly I also didn't mind. Perhaps this is because I felt that it may have taken longer to create then it did for me to discard it. My biggest objection to a whitelist-only system is that although emails out of the blue are very rare, they are also far more valuable than regular emails that I receive. Given my reaction above, I'd prefer a proof-of-work requirement over a blanket block. This still presents an undesirable requirement on the sender, but it would be better than a block.