I have some fairly strict email policies on our outgoing smtp relays at work yet one of them still managed to land on the following Yahoo blocklist. It's interesting that they are permanently using a temporary error condition. The only logic that I can think of for that is to allow a mail systems admin to check their queues and see what is in there before it bounces back to the original senders. More information from Yahoo follows below.
421 4.7.1 [TS03] All messages from x.x.x.x permanently deferred
421 4.7.1 [TS03] All messages from x.x.x.x permanently deferred
This error message indicates we are seeing a high volume of messages from your IP address that is a characteristic of unsolicited, bulk emails. While this is a temporary SMTP error code, we do not recommend attempting to resend your messages until you examine your subscription practices and lists, and implement changes to ensure that your messages are sent only to users who have requested it. Please visit this page to review our best practice recommendations.
If you have deployed significant changes or feel you have received this message in error, you may contact our Postmaster team by filling out this form.