To combat targeted spam to it's many customers, Yahoo has implemented a system whereby incoming email is delayed using SPF (Sender Policy Framework). For servers that publish SPF records, email is forwarded without delay. For originating IP addresses that are not listed, email is delayed for further processing. If the system is not altogether ill-conceived, then it certainly is very poorly implemented.
SPF was first implemented using TXT records, and later on a type 99 record number was assigned for that specific purpose. Our own DNS does not support type 99 records, but we do publish a TXT record which essentially says that we do not offer email forwarding service from any IP address (v=spf1 -all).
Spammers are using fictional user names and our domain name to send spam to Yahoo customers. Yahoo is then bombarding our DNS server with thousands upon thousands of requests for a type 99 record. This is repeated from the many internal DNS servers that Yahoo uses.
In order to keep our system operational, we have blocked all but one address from each group. We have sent many complaints to Yahoo pleading with them to address the situation, but so far it has fallen on deaf ears with the only response being automated.
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