Email, comment, & Post notification fail

Hi:
I have used WP for two months, and it was working great. However, 4 days ago, I noticed that all my email functions are not working anymore. From Contact form, comment notification, and to post notification…etc, they all fail to deliver any mails. I have some subscribers, and I blieve they are not receiving my recent postings like I don’t. I visited other threads regarding similar topic, they seemed all dug into php mails. My webserver is yahoo small business, and I checked settings were all fine. I even install another new blog to try if I could send emails, and it couldn’t either. Can someone kindly help? Please feel free to register to my this blog, and you can even find out that you do not receive your password email through registration. http://www.artificialhumanity.biz/blog1/