We use a scrip for Mighty Blue to extract a List of IP's from SPF records and put the in a whitelist. This has worked well for us. You can get it here: http://www.mightyblue.com/products.php?pid=5
Here is the list we currently import. Hosts_To_Check=apple.com,aol.com,google.com,live.com,paypal.com,dstwaterjet.com,nwmls.com,craigslist.org,ebay.com,msn.com,topproducer.com,usaa.com,becu.org,wf.com,amazon.com,mailchannels.net,myalaskaair.com,outlook.com,wahbexchange.org,frontier.com,eNom.com,comcast.net,edwardjones.com,ssa.gov,usps.com
Kendra Support
http://www.kendra.com
support@kendra.com
425-397-7911
Junk Email filtered ISP