Simple script to create white list of the mail servers of big companies who won't comply with the RFCs — for obvious reasons.
Grey listing is a very effective anti spam mechanism. The RFC states that any message that is not accepted should be tried again in a few minutes. Usually after a second attempt a grey filtering host accepts the message. Spam bots don't have time to go through those procedures and will not retry.
On the other hand, big companies like gmail and hotmail have to deal with massive volumes of mail and also don't retry sending emails.
To create a whitelist for those hosts this script checks their usually well maintained spf record. An spf record is stored in the txt record for dns.
Here is a complete syntax description for spf records: http://www.openspf.org/SPF_Record_Syntax