Over the last week or so a number of people have mentioned problems with delivery to Yahoo. It seems that some emails are being erroneously rejected. Earlier this week, Yahoo posted a message to the Yahoo Postmaster announcement list saying they were aware of the problem and were working on fixing it.
some senders are seeing intermittent IP blocks when sending to Yahoo! Mail, with the SMTP error message from us citing that the block was due to a Spamhaus listing — e.g., “553 5.7.1 [BL21] Connections not accepted from IP addresses on Spamhaus PBL.” (See our full list of SMTP error messages at http://postmaster.yahoo.com/errors/ .)
If your IPs are currently not listed on any Spamhaus blocklist but you are seeing this error, please be assured that we are looking into the matter. We shall post an update once we have resolved the issue.
What I am hearing is that this is not a consistent error. Reattempting mail rejected with this message may be acceptable practice until Yahoo fixes the problem.