Justin Wilson wrote:

We've been having trouble sending to *@yahoo.com Getting the infamous 421 Message from (x.x.x.x) temporarily deferred - 4.16.50. Please refer to http://help.yahoo.com/help/us/mail/defer/defer-06.html.

I've actually had quite a bit of success with Yahoo. They're slow in replying so you have to know what to ask for. Ask them to add you to their white-list. Once the techs respond, they'll send you a long questionnaire about your mail servers, management practices, and company policies. Fill it out and mail it back to them. You'll be put on a two week monitoring period where they'll monitor e-mails coming from your servers. At the end of the two weeks, they'll approve you if you are legit. Just grin and bear it. Include all relevant information in *every* e-mail you send them. You'll most likely talk to someone new every time. If you're patient and craft well-worded e-mails, you should be on their white-list in about three or four weeks. I did exactly this and so far the yahoo mail servers haven't been too troublesome. They slow down the connections sometimes, but it usually goes through after an hour or two. -Chris