We've been seeing delays of up to a couple of days on our outbound mail
to Yahoo.com for most of this month.  Last night I turned on logging and
raised the thread limit from 1 to 6 and then 12, and cleared off around
1300 messages to 14,000 recipients.  What I was seeing in the log was a
substantial number of "%SYSTEM-F-CONNECFAIL" and "%SYSTEM-F-LINKDISCON"
and some responses to "Mail From:" of:

>>> 421 mta619.mail.yahoo.com Resources temporarily unavailable. Please
                              try again later.

I'm guessing that YAHOO's individual engines are looking at the amount of
traffic they are getting from single sources and are, uh, load limiting?
Trying to block spammers?  What ever happened to the philosophy of making
the delivery of the mail the top priority?
                                             -Kary