[Exim] failed to find hostname..

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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)
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