I see. So when one Yahoo mail server rejects a message for "unusual volume
or user complaints", exim repeats the attempt with another Yahoo server.
When a mailing list delivery to several thousand recipients (around half of
them at Yahoo serviced addresses) somehow triggered Yahoo to start blocking
We're having a weird problem with which our VPS hosting company (HostGator)
has not been helpful so far. After discovering that Yahoo was blocking our
outbound email, I discovered an anomaly in our exim_mainlog which appears
to be exim sending retries for a failed message with no delay in between.
I missed the max_hosts_retry. Sorry.
For other cPanel/exim users experiencing delivery problems to Yahoo, I
found this article "1 Big Reason Yahoo! Defers Mail from cPanel"
(https://www.rackaid.com/blog/yahoo-defers-cpanel-mail/). It explains how
one deferral from Yahoo can blow up into a much big
I posted this problem as an exim bug, but it was immediately dismissed
because the MTA rejection response indicates a temporary problem, so
apparently retrying 4 more times inside a second to different IP addresses
for the same domain is deemed acceptable behavior rather than waiting a
normal retry
