My cow-orkers at EFF, Annalee Newitz and Cindy Cohn, have co-authored a brilliant white-paper on how spam-fighting is endangering the ability of nonprofit mailing-list operators to send email to people who ask for it.
MoveOn.org is a politically progressive organization that engages in online activism. For the most part, its work consists of sending out action alerts to its members via email lists. Often, these alerts will ask subscribers to send letters to their representatives about time-sensitive issues, or provide details about upcoming political events. Although people on the MoveOn.org email lists have specifically requested to receive these alerts, many large ISPs regularly block them because they assume bulk email is spam. As a result, concerned citizens do not receive timely news about political issues that they want. Often, MoveOn.org's staff doesn't discover that the mail isn't getting through for days or weeks, and even when it does, ISPs respond slowly to "unblock" requests or refuse to explain why email has been confiscated. Although ISPs may have the best of intentions, what we see in this scenario–one that is all too common–is free speech being chilled in the service of blocking spam.
In their zeal to stop spam, many organizations and companies are blocking the delivery of wanted messages, especially those sent through email lists. This problem is exacerbated by the fact that most blocking processes are not transparent to the email sender or recipient, and email users are generally given little or no control over which emails are blocked. Instead, system administrators, creators of spam-blocking tools, and ISPs all too often attempt to predict what mail a recipient does and does not want. As a result, email users rarely receive all legitimate messages sent to them.