Hi Bob,
I do not see that error being recorded by my SMTP relays today.
Which Yahoo mail server IP address(es) are you getting the error from?
I get the following DNS result for the Yahoo MX records as of about 12:15pm EST today :
a.mx.mail.yahoo.com. 1800 IN A 67.195.168.31
b.mx.mail.yahoo.com. 1800 IN A 74.6.136.65
c.mx.mail.yahoo.com. 1800 IN A 206.190.54.127
d.mx.mail.yahoo.com. 1800 IN A 209.191.88.254
e.mx.mail.yahoo.com. 1800 IN A 67.195.168.230
f.mx.mail.yahoo.com. 1800 IN A 98.137.54.237
g.mx.mail.yahoo.com. 1800 IN A 98.137.54.238
h.mx.mail.yahoo.com. 1800 IN A 66.94.236.34
Try a telnet to each of their IP's and see if you get the error from one, several, or all of their mail servers.
Is there any indication that one of your clients recently spammed Yahoo?
Do you have the ability to slow down how fast and how many concurrent connections you have open to the Yahoo Mail servers at the same time?
I do not see an announcement in the Yahoo Postmaster group saying there are known problems with their servers today :
ymailadmin : ymailadmin
If you have not already you should let Yahoo know the functions for your SMTP Relays and their IP addresses (X servers send normal mail, Y servers send bulk mail, Z servers send email forwarded from pop account hosting, etc) using the Yahoo Bulk Sender Form :
Yahoo! Mail Bulk Sender Form | Yahoo! Postmaster Help
I hope this helps and welcome to Email Questions!
-Raymond