One of the biggest reasons I support yahoo is for its spam detection capabilities and its efforts to reduce spam in this world with Yahoo! DomainKeys technology. Some rough observed statistics: Yahoo correctly identifies the 80-some spam I get everyday and throws them in the 'Bulk Mail' folder. I mean its almost precise. Once or twice a week I get to see one or two spam mails that escaped detection and ended up in my inbox. Thats a pretty good job on Yahoo's part. Also, there have been very few (almost none) false positives - mail that was not spam, but yahoo identified it as spam - its so good that I haven't had any false positives since the last 2 months or so.
Also, one of Yahoo's anti-spam efforts is to use captchas to prevent unscrupulous advertisers from creating new free accounts and spamming the hell out of everyone. A captcha is an image with squiggly letters, which you have to identify and enter to prove you are a human and not a bot, that we often get while registering for a new account on many sites. I know that sometimes when you send mail, especially to multiple recipients or groups, Yahoo asks you to verify using a captcha to ensure you're not spamming.
However these days, Yahoo captchas me on every single mail I send. Even when I am corresponding to people within my address books. Whats happening? Its ridiculous!
Also, one would think that their automatic learning algorithm (that learns things specific to your mailbox/email activity), where if you've exchanged a few mails back and forth in a conversation, the next time you reply to an incoming mail, it'd be reasonably sure that its not spam. But these days I get captchas even if its the 3rd or the 4th mail in a conversation and I'm replying to a previous mail sent by the person I am writing to.
I have no clue what has changed and from the little information I have, my conclusion is this: Either someone has been spoofing my email address and spamming the hell out of a ton of yahoo users. Hence Yahoo is suspicious and has flagged my account and captchas all my outgoing mail. If its not that, something is seriously wrong with their spam checking algorithm. Either way, its getting very annoying. I hope it gets resolved soon, or else I'm moving to Gmail. One can only have so much loyalty!