A secure democratic Net is a fundamental right

SPF (Sender Policy Framework) is a sender authentication method through which the administrator can specify which hosts are allowed to send email on behalf of a domain. The sending domain adds a specially formatted record into its DNS zone file identifying all authorized mail servers. The receiving mail server checks the sending domain's DNS zone file to see if the IP address from which the message originates matches one of the authorized IP addresses.

and you can do it the simple way here http://www.mailradar.com/spf/

this is only one way to protect against phishing and spam. It doesn't harm to do it. In Belgium very few mailserver have any ID system on their mailserver.

This makes it easier for others to send emails with their domainname from infected computers or hacked or abused mailservers.