For reasons unknkown to us, Yahoo! started blocking all mail from antiflux earlier this week. Anything sent from antiflux to a Yahoo! address is blocked but won't bounce until maybe a few days later when the antiflux mail server gives up.
Looks like it started last Sunday morning.
Jun 28 08:56:33 okcomputer postfix/smtp[20779]: 1E8BA38F9E: host b.mx.mail.yahoo.com[66.196.97.250] refused to talk to me: 421 4.7.1 [TS03] All messages from 216.234.161.200 will be permanently deferred; Retrying will NOT succeed. See http://postmaster.yahoo.com/421-ts03.html
(That URL is wrong. It's actually here.)
There's no automated way to find out why they're blocking mail from us and they seem to be ignoring the query I sent. I started the process to sign up for their Complaint Feedback Loop but they won't do anything until we start signing outgoing mail with DomainKeys/DKIM. We're looking into that.
Update (July 5): Yahoo! seems to be accepting antiflux mail again, with no clue as to why they changed their minds. After a series of emails back and forth with their support department, I still have no idea why they were blocking us. Either they don't know either or they just refuse to give out any information. Their suggestions ranged from us being listed on a Spamhaus block list (we aren't) to us sending a "high volume" of emails "indicative of unsolicited mailings" (we weren't), but they refused to give any kind of definitive statement of why we were actually being blocked. Seriously, if you use Yahoo! Mail and want to actually receive email, I strongly suggest that you move to Gmail or someone else that doesn't randomly block mail servers.