Microsoft has a nice wizard available that will create a full proof SPF record for your domain.
How does Sender ID Framework work?
- Sender sends an e-mail to Receiver.
- Receiver’s inbound e-mail server receives e-mail and calls its Sender ID Framework.
- The Sender ID Framework looks up the SPF record of the domain that Sender is using for sending the mail.
- The receiving Mail Transfer Agent (MTA) determines if the outbound Mail Server IP address matches IP addresses that are authorized to send mail for the user.
You can find the simple 4 step wizard here: http://www.microsoft.com/mscorp/safety/content/technologies/senderid/wizard/