SPF stands for Sender Profile Framework. SPF records provide a way to indicate what servers you use for outgoing email, thus allowing email receivers to verify that the email came from an expected source thereby reducing spam. SPF records are stored in text records (more on TXT records in a future post) however in DNSimple SPF records are considered their own type of records. DNSimple provides an easy interface for creating SPF records so you don't have to manage those records on your own.
SPF records include several pieces of information that are used to verify the sender of an email. First you can indicate whether email can be sent from the IP address of the domain. For example, if you have an A record for example.com pointing to 10.0.0.1 your SPF record can indicate that email is sent from this server. In DNSimple this is as easy as checking the box next to "I send mail from this host". When an email recipient receives an email from you and the originating address is 10.0.0.1 then it will know that the email came from you and thus should not be marked as spam. You can also indicate the same for MX records by checking the "I send mail from this host's MX servers". In this case if email originates from one of your MX servers then the client will be able to validate that it is valid.
The IP Networks field allows you specify networks from which you send email. IP Networks are defined as address blocks. For example, 192.0.0.1/32 indicates that email will come from the IP address 192.0.0.1 only. On the other hand, 192.0.0.1/24 indicates that email will be received from anything from 192.0.0.1 to 188.8.131.52.
Finally, the Include field allows you to include SPF rules from another server. An excellent example of how to use this field comes from Google Apps. In order to use Google's SPF record you simply set the value of the "Include" field to
_spf.google.com. This will automatically use the Google SPF record for your SPF profile.