Rule number aleph-null: If you would like to unsubscribe from an e-mail list to which you have opted in, follow the instructions you were sent when you subscribed and which are mailed to you periodically, often at the bottom of each message. Do not click "This is spam" in a webmail client, especially if your e-mail service provider is Yahoo, Gmail, or some similar broadly-subscribed national service.
One too many boorish dummkopfs (yahoos?) using Yahoo mail told their client that Randy Cassingham's This Is True was spam instead of unsubscribing properly. As a result, tens of thousands of subscribers--many of them subscribers to the paid "premium" version--did not receive their newsletter for several weeks.
In the last newsletter, Cassingham reported that after a lot of back-and-forth with Yahoo support, his e-mails are now reaching subscribers and customers. However, there's no doubt that losing 15% of his subscribers for several weeks hurt Cassingham's business.
The lessons: Internet users should take spam reporting seriously, and service providers should ask "are you sure this isn't an opt-in list?" before accepting the user's click. This sort of thing has happened before. I was webmaster for an aquarium club, using the free Crosswinds service as host, when the site was blackholed because a spammer faked a Crosswinds origin for the e-mail, meaning members couldn't access the site for weeks. If the reporters had checked the headers, as is good practice, they'd have found that it was sent from an open relay elsewhere.