[Exim] failed to find host name from IP address

This is a multi-part message in MIME format.
--
[ Picked text/plain from multipart/alternative ]
What does this error msg mean?
I get allot of them...

2002-08-28 00:16:21 recipient <abxxadxxn@???> refused from (hotmail.com) [200.27.89.146] sender=<5rdoi3xg@???> (failed to find host name from IP address)
2002-08-28 00:17:17 recipient <mikael.wendt@???> refused from (relay.enpia.net) [211.106.159.46] sender=<almohamed@???> (failed to find host name from IP address)
--