Howto setup SPF (Sender Policy Framework) on a domain

SPF is a Policy Framwork that helps fighting return-path address forgery and makes it easier to identify spoofs. Domain owners identify and pinpoint sending mail servers in a DNS record, and thereby its posible for SMTP receivers (e.g. MTAs like Exim, Postfix, Qmail etc.) to verify the envelope sender address against this information, and can distinguish authentic messages from forgeries before any message data is transmitted.

Create a SPF record
The easist way to create a SPF record is to use this online tool: openspf

Deploy the SPF record
To use the newly created SPF record on a domain, make sure you have access to create a TXT-DNS record for the given domain. If you have access to create a TXT-DNS record all you need is to create such a TXT-DNS record containing the SPF record information, and you are done.