If you are seeing large outbound queue in your email server to Yahoo with following error, there should be a problem from Yahoo MTA:
420 Resources unavailable temporarily. Please try later (mta1392.mail.bf1.yahoo.com)
Yahoo had similar issues in the past.
Ref: http://tech.groups.yahoo.com/group/ymailadmin/message/17
As soon as Yahoo fixes their MTA problem, all your emails should be delivered.
