Reverse Lookups fail

My domain's reverse lookups are succeeding when making queries directly
to the DNS server, although from anywhere else in the world, they are
failing. For example:

bash-2.05b$ /usr/sbin/nslookup 206.135.58.110 dns1.mindbridge.com
Server: dns1.mindbridge.com
Address: 206.135.58.120

Name: mail2.mindbridge.com
Address: 206.135.58.110

bash-2.05b$ /usr/sbin/nslookup 206.135.58.110
Server: noc.irt.drexel.edu
Address: 144.118.24.20

*** noc.irt.drexel.edu can't find 206.135.58.110: Non-existent host/domain
bash-2.05b$


All other types of lookups are succeeding from other DNS servers. Does
this indicate that our ISP is not delegating reverse lookup authority to
us?


This is the zone file, cut down a little...

$TTL 3H
mindbridge.com. IN SOA dns1.mindbridge.com.
support.mindbridge.com. (
2005040801 ; serial
1D ; refresh
4H ; retry
6W ; expiry
1W ) ; minimum

mindbridge.com. IN A 206.135.58.101
mindbridge.com. IN NS dns1.mindbridge.com.
mail2 IN A 206.135.58.110
....


Thanks for any help

Ted Watson