We're seeing large backlogs of outbound mail to yahoo.com mail servers right now. Expect delays in delivery up to 5 hours if you are sending mail to recipients with yahoo.com mail addresses. Mail is getting through (though yahoo is also arbitrarily rejecting some mail as well) just very slowly.

It is pretty common for us to see delays getting mail to Yahoo, but today seems particularly bad for some yet unknown reason. Unfortunately there is not much we can do since the issue is on their end. Please be patient while the queues clear.