It's because users need to validate their email when they register their accounts My webhost said : "Yahoo has blocked your server for the amount of mails that are being sent through it." Any idea how to solve this ?
Try http://help.yahoo.com/l/us/yahoo/mail/yahoomail/index.html Fill up the necessary form and expect it to take about a week, they may even email you another questionaire with about 17 questions.