From yahoo, on 29/11/2004 (DD/MM/YYYY):

[root@veneer.qala /]# telnet mx4.mail.yahoo.com 25
Trying 206.190.36.244...
Connected to mx4.mail.yahoo.com.
Escape character is '^]'.
421 VS1-IP Excessive unknown recipients - possible Open Relay http://help.yahoo.com/help/us/mail/spam/spam-18.html (#4.1.8)
Connection to mx4.mail.yahoo.com closed by foreign host.


http://help.yahoo.com/help/us/mail/spam/spam-18.html states:

As a sender, how can I ensure uninterrupted SMTP access and prioritized delivery?

Yahoo! Mail has become more aggressive in its acceptance of SMTP connections and denies connections by IP address when these connections do not conform to Internet standard practices. To continue uninterrupted access and prioritized delivery, you should ensure that your email address lists are well maintained. If you are conforming to these standards, then these policies should only affect you positively. We anticipate improvements in delivery times and in available connections.

If you consistently see 421 SMTP error codes you should review your email practices and policies, especially the following:

    * Remove email addresses that bounce. Bounces are an indication that the mail could not be delivered because the user does not exist, no longer exists, or is unable to accept your email. List managers should remove addresses that generate bounces. A particularly popular technique for managing bounces is to use VERP to identify the recipient address that has failed.
    * Pay attention to the responses from our SMTP servers. The SMTP protocol defines response codes that tell your server what to do next. In particular, our server will send "500" SMTP response codes to indicate problems that you need to investigate. For example, if an email is sent to an invalid recipient, our servers will respond with a "500" range SMTP code, indicating a permanent error. Permanent errors should not be immediately retried. Many companies following best email list practices treat email addresses that have received a "500" range SMTP codes specially. For instance, many companies place these addresses into a special queue that tests for continued bounces after longer and longer periods of time. This helps them maximize delivery to email addresses that exist and minimize wasted bandwidth costs.
    * Don't send unsolicited email. Make sure that all email addresses are confirmed with an opt-in process that ensures the recipient wants to receive your mail. Obtaining permission from a third party to send an email does not ensure the email is solicited. Probably the best way to confirm an email addresses before adding them to a mailing list is by using closed-loop confirmation (sometimes refered to as "full confirmation," "full verification," "confirmed voluntary subscription," or "double opt-in"). In this process, after you receive a subscription request, you send a confirmation email to that address which requires some affirmative action before that email address is added to the mailing list. Since only the true owner of that email address can respond, you will know that the true owner has truly intended to subscribe and that the address is valid.
    * Provide a method of unsubscribing from your list in each mail you send.
    * Ensure that your mail servers are not open relays, and that your servers attempt to detect and deny connections to open proxies. At a minimum, your SMTP servers should identify the originating IP addresses that of the email and indicate this in the email headers to help you diagnose spam problems. There are a variety of open relay testers available. 

If you conform to the above practices and are still seeing delivery problems, please fill out this form so that we can help you diagnose your problem. Please do not use this form to report spam