If you are an ISP or institution with a lot of customers who like to inflict pain on themselves by forwarding all their email to the free Yahoo email service, this is an important tip for you.
The problem reached crisis proportions since a majority of the 1500 or so staff and faculty at the place where I work are forwarding their mail to free Yahoo addresses. (I have no control over this.)
July 2009 Update: Yahoo has introduced intrusive, annoying, popup ads in their free email. My users are fleeing Yahoo in droves and going to Gmail, where they should have been all along.
As of the last few weeks Yahoo is now accepting only FIVE messages per SMTP socket connection, then they hang up on you, which your MTA will probably interpret as a failing connection. Unfortunately, the tendency for most MTA's is to back off on failed connections, which only makes the Yahoo problem worse.
The latest round of Yahoo postmaster docs (http://help.yahoo.com/l/us/yahoo/mail/postmaster/) invites you to open more than one SMTP connection, and retry as often as you like if you get 421's, up to some unspecified limit. When I discovered this change in behavior, I had 2400 emails destined for Yahoo stuck in my queue, and I was able to flush them all out in one hour by upping the number of outgoing SMTP sockets from 3 sockets per domain to 15 and continuously kicking our MTA repeatedly with the "clear out the queue" command, and all of them got sent.
So, unlike most email services which probably encourage you to make as few SMTP connections as possible, and stream multiple messages per socket, the approach with Yahoo is to keep banging away one to five messages at a time. Your MTA probably allows you to tune down the lifetime of SMTP connections. Unfortunately, no MTA I know of allows you to tune this on a per domain basis, but, usually, just repeatedly processing the queue is sufficient, since 90% of your stuck messages will be for Yahoo. You can even write your own simple MTA to just pick out messages destined for Yahoo, send them, and delete them from the "real" MTA's queue.
The behavior is only present in the free Yahoo email service; ymail.com, sbcglobal.net, etc, are not affected.
You want to be somewhat careful about this, since in the unfortunate event you get 3wned and then blacklisted by Yahoo or some other provider, you don't want to hammer away blindly at your queue if it actually is full of spam.
As for availabilty of the Yahoo servers for connections, I am logging this carefully, and at this time about 1 in 10 of my SMTP connection attempts to Yahoo actually do fail to get a HELO within 15 seconds of connecting.