I have 1 email address for correspondence and 8 others that I share with my wife for credit cards, bill payments, etc. All 8 email addresses are through one ISP that has a very effective spam filter (Brightmail) that is server based. It's a very good filter, using whitelists. I can approve incoming email addresses, or if people email me using a non-whitelisted address, the filter sends them back an email asking them to confirm their email address. They must do so by using by key entering a word contained in a graphically distorted image into a box (the de facto standard in online verifications). Brightmail also uses confirmed blacklisted IP addesses of known spammer sites.

I've been doing this for 3 years. It is very effective and I have very little spam that I personally have to deal with, even though my main email address receives approximately 30 spams per day that I never see.

I change my remaining 7 email addresses frequently - every 2 or 3 months. If I ever receive any spams to these addresses, I delete them and replace them with new addresses. Yes, it's extra work, especially because I have to go into my credit card websites and redo my profiles. But it's worth the extra measure of privacy.