For the past few weeks, I've been trying to email users at Yahoo.com.  Their MTAs continuously reject email I send:

421 Message from (209.161.207.163) temporarily deferred - 4.16.50.
Please refer to http://help.yahoo.com/help/us/mail/defer/defer-06.html

I've already put in reverse DNS entries, so cairo.justinho.com <--> 209.161.207.163 both resolve, whichever way you try.

I'm not even sending spam.  I'm trying to send emails to friends or responding to friends' emails.  I had a Craig's List post a few days ago - and I was responding to a Yahoo user.  But it's no use - I still can't email Yahoo.com users.

I've contacted Yahoo tech support lots of times.  I've requested to be "unblocked" - heck, I've even filled out their form.

Why is it that I somehow need Yahoo's permission to send email to their users?  Why do they even NEED my contact information?  Two weeks since sending all this information they wanted, I STILL can't email @yahoo.com email addresses.

This is ludicrous.  If you are a Yahoo.com user, I strongly suggest you get an email account at another provider.

Applying for Permission to Email Yahoo.com Addresses

1. Please provide all ACTIVE IP addresses you are currently using to send mail.
2. Do you have a dedicated IP address or do you use a shared mail server/IP address (i.e., the mail server/IP address is hosted by a service provider and is also being used by organizations other than your own)? If you use a shared server, please specify which service provider you use.
3. Please indicate all types of email being sent from the servers above, e.g., personal/corporate emails, transactional mailings, mailing list postings, marketing messages, newsletters? (Indicate which IP sends which type of mail, if applicable.)
4. If you send periodic or subscription-based mailings, please indicate the means by which a user is signed up for your subscription list. 
   1. Do you take any steps to confirm that the subscription is valid, or was initiated by the true owner of the email address?
5. How many subscribers do you currently have? And approximately how many emails do you send on a monthly basis?
6. Do you remove email addresses from mailing list if emails to them bounce--i.e., for soft (4xx) and hard (5xx) SMTP response codes?
   1. If yes, how many bounced emails are required before you consider an email address to be inactive and subject to removal from your list?
7. How long does it typically take for an email address to be removed from your list once an unsubscribe request has been received?
8. Please provide the URL of your web site, including the links to your Privacy Policy, Affiliate Policy, and/or Terms of Use pages, if available.
9. Please copy and paste a text-only example of a recent mailing, including the full Internet headers. Or, include the entire error message you're seeing in your SMTP logs if email is being deferred or blocked.
10. Where possible, Yahoo! uses DomainKeys to determine the original sender of a message. Do you plan to or currently use DomainKeys to authenticate your mailings?
11. Please provide the following contact information:  Company name, Contact information, Postal Address, Email address

When was the last time you applied for permission to send email to someone?