negative caching of throwaway spam domains

[Ken A]

Hi,

We have 3 spam filtering machines that each run a bind caching 
nameserver to help with rbl lookups, etc..
After mail passes through these machines it goes to our mail hub.

Every so often, a spam from a throwaway spam domain will get through the 
spam filtering machines to the mailserver hub. The caching nameserver on 
the spam filtering machine will be able to lookup the sender's hostname, 
so sendmail accepts it.

But, sendmail, on the mailserver hub will bounce it back to the spam 
filtering machine with an error.. 'Domain of sender address 
jthlhiyue at halosalbum.com does not exist'. (that one is from this am.. 
registered yesterday by a spammer).

The question is, is there something I can do to, other than telling the 
mail filter machines to all use the same instance of bind to avoid this 
happening?

Also, a bit off topic, but it occurs to me that this kind of information 
is useful in spam fighting. Are there any rbls out there that list all 
domains registered in the last 48 hrs?

Thanks for any ideas!

Ken A
Pacific.Net