Earlier quoted context omitted.
Not using block lists will inundate mail servers with spam. It's a necessary evil.
I think that gray listing is a great idea to filter out spam. Here's how it works: the very first time an email server get an email from a new IP it answers with a "temporary unavaiable" error. According to specs, a server should retry to deliver the email after a while, so a legitimate server will retry and it's IP will be put in a "withelist" (here in quotes because you can still do further processing of the emails…
That is the reason why I switched on some external block lists into the mix.