Why Yahoo Can't Deliver Email

I'd been wondering about this. In recent months, I've tried to get email into or out of my Yahoo! Mailbox many times, without success: "The web is alive with complaints from Yahoo Mail account holders who didn't get mail, and legitimate senders whose mail never was delivered. Everybody seems to know it happens, but nobody ever figured out why... until now. After Email Battles mapped, analyzed and charted Yahoo Mail's mail servers, it's easy to see why the wealthy company wants to charge for certified sender services. It apparently needs the money for a new mail server."