Hello Patrick,
  
  Thank you for writing to Yahoo! Mail.
  
  We appreciate your report to Yahoo! Mail's Postmaster Group.
  
  If you continue to see persistent 4xx errors in your SMTP logs when 
  delivering email to Yahoo! Mail accounts, there are several factors that
  may be causing this issue:
  
  1) Your server is sending an unusually, higher volume of email traffic 
  to Yahoo! Mail: When we see a significant increase in email volume 
  coming from a server (as compared to its historical volume), we may 
  defer connections from that server temporarily.
  
  2) Mail coming from your server are generating complaints from our 
  users: Yahoo! Mail constantly monitors feedback from our users and we 
  use that feedback to feed into a sender's reputation. If our users 
  report your mail as spam on a consistent basis, your server's chances of
  getting connection deferrals are elevated.
  
  3) A combination of (1) and (2).
  
  While these 4xx deferrals are temporary, they should be treated as a 
  notification for you to examine your outbound queue and/or the content 
  of your outbound mailings.
  
  FOR BULK MAIL SENDERS
  
  We urge you to review our recommended best practices targeted at bulk 
  senders like you at the URL below:
  
  http://postmaster.yahoo.com/basics/postmaster-15.html
  
  If you adhere to these practices, please click on the URL below and fill
  out our Help form so we can evaluate your mailing practices, and 
  determine if you are eligible for prioritized delivery to Yahoo! Mail.
  
  http://postmaster.yahoo.com/bulkv2.html
  
  FOR ISPs/HOSTING PROVIDERS/OTHER MAILING ENVIRONMENTS
  
  For servers that are sending consumer email, please provide us with the 
  information requested below so we can investigate your issue and 
  possibly make changes to your server's classification in our systems.
  
  a) Detailed description of the problem (including when it started)
  
  b) The sending IP(s) affected
  
  c) What types of emails are being sent from the affected IP(s)
  
  d) The specific error code(s) your servers are getting from our MTAs (if
  applicable)
  
  We appreciate your patience as we try to address your delivery issues to
  Yahoo! Mail. 
  
  Thank you again for contacting Yahoo! Mail. Your case number for this 
  issue is 69027013. Please reference it in all future communication about
  this particular issue. 
  
  Regards,
  
  Hank
  
  Yahoo! Customer Care
  
  69027013
  
  
  
  
  Original Message Follows:
  -------------------------
  
  Mail-Id: 
  w1.help.re1.yahoo.com-/l/us/yahoo/mail/postmaster/defer.html-1267628743-
  201
  
  1.  What is your name?
  ----------------------
  Name: Patrick Fanning
  
  2.  What is your email address?
  -------------------------------
  Email Address: patrick@ancc.net
  
  3.  What domain(s) are you contacting us about?
  -----------------------------------------------
  Domains: mail.xxxxx.com, mail.xxxxx.com
  
  4.  About the problem:
  ----------------------
  Subject: Experiencing delivery delays
  
  5.  Indicate the specific IP address of the mail server experiencing the
  issue.
  
  ------------------------------------------------------------------------
  -------
  208.71.18.xxx
  
  6.  Enter the full text of the error code(s) your mail server received 
  from Yahoo!:
  
  ------------------------------------------------------------------------
  -----------
  Reason: Remote host said: 421 Message from (208.71.18.202) 
  temporarily deferred - 4.16.50.
  
  7.  Copy and paste the contents of the email here:
  --------------------------------------------------
  Not set by user
  
  8.  Enter additional information here:
  --------------------------------------
  One of our multiple domain names, but NOT sharing IP addresses
  
  
  "Information Passed In":
  ------------------------
  "from_url" : http://help.yahoo.com/fast/help/us/mail/cgi_defer
  "last_url" : http://help.yahoo.com/fast/help/us/mail/cgi_defer
  ------------------------
  
  While Viewing: http://help.yahoo.com/fast/help/us/mail/cgi_defer
  
  Last URL: http://help.yahoo.com/fast/help/us/mail/cgi_defer
  
  Form Name: http://help.yahoo.com/l/us/yahoo/mail/postmaster/defer.html
  
  Yahoo ID: 
  
  Other ID: 
  
  Machine: PC
  
  OS: WinXP
  
  Browser: Mozilla 1.9
  
  REMOTE_ADDR: 208.71.16.30
  
  REMOTE_HOST: FAST.ancc.net
  
  Date Originated: Wednesday March 3, 2010 - 07:05:43
  
  Cookies: enabled
  
  AOL: no
  
  -------