What is SPF ?
SPF means Sender Policy Framework. It allows the domain's owner to make a list of servers allowed to send messages on its name. This procotol protects sendings made on your name.
How does it work ?
When a server receive a message, it verifies in the domain's TXT records that the sender is allowed to send or not this email.
If the message come from a permitted server, then the message will be legitimated. Otherwise it will be rejected.