I wrote a new article this weekend that identifies the main reason that Yahoo temporarily defers mail messages with an error 421. With spam mail messages on the constant rise, we can hardly blame Yahoo for being more strict on blocking mail messages that aren’t guaranteed to be spam free. With this problem, Yahoo has a solution for businesses that have to send commercial or bulk email- DKIM. DKIM solves Yahoo’s spam problem by allowing them to authenticate emails sent from your server, and it solves a businesses’ problem of ensuring that important non-unsolicited bulk email is delivered.
Many administrators and email marketers alike have been frustrated by the addition of SPF and DKIM authentication, and few know how to properly tackle this burden. As always, if you need help installing a DKIM filter, or adding SPF to your domain’s DNS records, be sure to contact me.
Here is the article:
http://linuxconsultant.info/tutorials/yahoo-temporarily-defers-email-with-error-message-421.html