Hello. can someone help me get this error and continuing server errors with DKIM?

The problem is that i am getting this in my OSSEC logs my the thousands from this ONE domain. Postfix, mailman. I have no other problems with DKIM ever, but for the last few weeks with this error. 

Received From: www->/var/log/maillog
Rule: 1002 fired (level 2) -> "Unknown problem somewhere in the system."
Portion of the log(s):

Feb 16 20:42:47 www dkim-filter[1305]: 560704ACD3D: key retrieval failed (s=k1, d=monobestoffers.info): res_query(): `k1._domainkey.monobestoffers.info' Unknown host

IT would not bother me, except i have blocked the ip from the firewall and continue to recieve the errors and am worried about the load on the mailsystem and server.

How can I stop it and what is going on, any help would be appreciated.