I'm email admin for a large organization, and I've been investigating a complaint from a user who tried to send from his work account to his yahoo account and got an NDR. (The message cannot be delivered due to a configuration error on the server. Please contact your Administrator. <my SMTP gateway #5.3.0>). My SMTP gateway event log said "Sorry your message to
xxx@yahoo.com cannot be delivered. This account has been disabled or discontinued [#102]. - mta141.mail.re3.yahoo.com"
I haven't been able to speak with the user directly to see if the account could be closed, so there could be a problem at yahoo. But in my testing I have been able to log on to my yahoo account and send test messages back and forth. So there may some users down, but the entire system is not down.