@Erick Stubbs: Including ptr in an SPF record doesn't seem like a good idea to me. Simply put, if I have control over my own DNS, I can add a PTR record of 1.2.3.4 to mxa.mail.google.com (or whatever Google's first SMTP server is). It's A records that are *generally* safe, as they map DNS names to IPs. The A record for mydomain.com is under my control, unless the root server for .com gets tricked or the receiver has had their DNS hacked (and then it's their problem, not mine).