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Date: Fri, 18 Jul 1997 00:55:21 -0700 (PDT)
From: Shawn Ramsey <shawn@luke.cpl.net>
To: Swee-Chuan Khoo <sckhoo@asiapac.net>
cc: questions@FreeBSD.ORG
Subject: Re: qpopper2.3
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> hi,
> 	i have a qpopper2.3 installed in a freebsd2.2.2 machine
> which is also running bind4.9.6. 
> 	i am wondering why this error mesg came out on the console
> and also popper.log file (local2.info).
> 
> Jul 18 14:14:50 luke popper[16181]: (v2.3) Unable to get canonical name of
> client, err = 0

This means it can't get the domain name of the IP that is using popper.
Try doing a nslookup on the IP, if it fails that is the reason.