Need more information - when dealing with DNS, information such as IP and host/domain names are needed.
If you are still having problems with mobot.org, then from what I can see, reverse lookup is not having a problem (although the answer may not be coming from where you want it to come from).
From a nslookup:
> set type=ptr
> 208.46.83.49
Server: myserver.mydomain.com
Address: 10.140.2.2
49.83.46.208.in-addr.arpa name=www.mobot.org
83.46.208.in-addr.arpa nameserver=sv1-ans-01.inet.qwest.net
83.46.208.in-addr.arpa nameserver=dca-ans-01.inet.qwest.net
>
> set type=any
>208.46.83.49
Server: myserver.mydomain.com
Address: 10.140.2.2
***myserver.mydomain.com can't find 208.46.83.49: Nonexistent host/domain
>
www.mobot.org
Server: myserver.mydomain.com
Address: 10.140.2.2
www.mobot.org internet address =208.46.83.49
www.mobot.org preference=0, mail exchanger=webmail.mobot.org
mobot.org nameserver=svl-ans-01.inet.qwest.net
mobot.org nameserver=gatekeeper.mobot.org
mobot.org nameserver=dca-ans-01.inet.qwest.net
webmail.mobot.org internet address=4.36.133.74
svl-ans-01.inet.qwest.net internet address=205.171.14.195
gatekeeper.mobot.org internet address=208.46.83.62
dca-ans-01.inet.qwest.net internet address=205.171.9.242
>
But I've found this to be an intermittent problem - and once my servers have it cached, I can't test as easily (can't bring down my DNS to assist).
