[CentOS] DNS lookup delay with centos & postfix

I'm a bit baffled by this and I'm looking for ideas...

background:
two DNS servers (ns1 & ns2)(64bit CentOS 5.8)
one email server (64bit CentOS 5.8 & postfix 2.3.3)
one nagios server (64bit CentOS 5.8 & nagios 3.3.1)

situation:
- all servers configured to use both DNS servers for lookups
- ns1 server down for hardware problem
- nagios alerts that smtp on email server taking longer than 2 seconds 
to respond
- nagios alert for smtp on email server clears when ns1 returns to service

- when I use dig from the email server command line there is no problem 
or delay when ns1 is offline.  It worked without a hitch using ns2.

Anyone have any ideas for why nagios would have trouble testing smtp on 
the email server when the primary dns goes offline?  I'm not even sure 
where to look or who else would make sense to ask the question of on 
this one.  I'd appreciate any insight anyone out there has on this.
-- 
Steve