An article from Websense shows how new bots are able to bypass the CAPTCHA security that prevents spammers from creating large number of accounts.
According to Websense the three main advantages for spammers to target this CAPTCHA are:
- the Microsoft domain is unlikely to be blacklisted
- they are free to sign up
- it may be hard to keep track of them as there are millions of users worldwide using the service.
The same arguments are true for the other major providers like gmail, yahoo, etc.