Glenn Fleishman has a thought-provoking essay in the Economist about overeager spam filters, and collateral damage in the war on unwanted junk email:
We should still be slightly behind the spammers, reading the small percentage of their most creative efforts that actually get through. And yet, from my own experience and stories I hear from fellow hoary internet veterans, something has broken. Many dozens of emails I've sent in the last year have never reached even a recipient's filtered folder. A few weeks ago, a note about compensation failed to reach the editor of this blog. (Yes, I believe him. Why do you ask?) Likewise, many messages never arrive into my inbox or spam folder. No rejection message arrives, to be decoded; no ham waits to be discovered among the spam. Mails are simply disappearing.
Your theories are welcome, but I believe that the complexity of getting through a spam-filter maze with ever more dead ends is a key cause.
The emerging ambiguity of e-mail (economist.com)