[CentOS] dns cache rbl lists?

No idea where else to ask this and get a real qualified answer but here.
Not exactly pure centos question....but...

I am adding blacklists to my postfix smtpd settings.
I have the inkling that after the first lookup for a domain or ip that 
my dns caches the result and I no longer bother the RBL or RHSBL list 
owners anymore in that instance.

Is that correct? I hate to waste their resources if it is not.

thanks,
bob