[Exim] failed to find host name from IP address

This is a multi-part message in MIME format.
--
[ Picked text/plain from multipart/alternative ]
Today I see in my logs an increased number of rejections because of
"failed to find host name from IP address". While this is perfectly fine
when dealing with spammers I have the uneasy feeling that exim sends a
reject upon every kind of DNS failure. (some of the DNSes I tried
managed to resolve the IP in question while others didn't).

Or can someone confirm that eg. upon DNS time-outs exim sends a 451 (?)
temporary error message instead of a 550?

--
Ben
--