SPF is a pretty cool system, but it's not perfect. It works very well for single servers but if you send mail for your domain from multiple servers (and perhaps a varying number of them), you are required to use wild-cards which reduce the effectiveness of this system.
=== DomainKeys Identified Mail (DKIM ) ===
One DomainKeys Identified Mail (DKIM) is one of several popular spam-prevention measure measures that uses use encryption to sign messages originating from your server so that receiving servers can verify that the messages really did come from your server (the keys are published in DNS).
It's a neat idea DKIM is considered to be an interesting method, but the strength of the encryption using DKIM is pretty patheticnot high. So it's DKIM is a good deterrent against mass amounts of spam, but it doesn't really guarantee anythingother types of attacks on your email server.
=== Address Books ===