rejecting senders with the above method is problematic, cause the reject decission is based on the first match of a single test, you should consider to implement:

 http://www.policyd-weight.org/

policyd-weight is calculating a score, based on different tests (HELO, MX, DNSBL, RHSBL) and only if the resulting score is passing a certain value the sender is rejected.

also policyd-weight is implementing a caching mechanism for the blacklist lookups