Yahoo either likes it that way, or is totally uninterested in fixing it.
Sometime this past weekend, the servers that receive mail for @yahoo.com addresses started deferring email from most IP addresses, as far as I can tell. For the geeky among you: Connect to any of Yahoo's MX's, and you get "451 Message temporarily deferred - 4.16.50" before you even get a chance to EHLO. For the non-geeky: Yahoo's basically saying "go away, come back later and try again", over and over and over, without even trying to find out what you're trying to send or who it's to. Ever since then, any email I've sent to an @yahoo.com address has just waited, deferred, for days. It seems that sometimes, on rare occasions, it gets through - but usually only after a day or longer.
Yahoo isn't doing this to everyone. I tried connecting to their mail servers from a number of places, and while I got immediate deferral from most, it didn't happen when I tried to connect from world.std.com. I'm not sure what exactly they're doing. Maybe they have a list of known major mail servers they allow through. Maybe they're giving special treatment to the companies that pay them for better email service (you might remember the brouhaha about that from this spring). Whatever they're doing, they're not telling, and they don't wanna talk.
I tried submitting requests through their online help form, twice. No response, days later.
I tried calling their corporate office, and their customer care number. I got dead ends.
They insisted I pay them for support or they couldn't help me. No transfers to managers (unless I know their full names). No messages for someone to call me. No tickets opened. Just a few hours wasted making phone calls.
If you use Yahoo for email, please leave them.
And if you want to get any emails from me, let me know what your non-yahoo address is.