While the SPF protocol allows for a number of detailed and technical applications, there are some fundamental mechanisms which are sufficient for the vast majority of cases.

The mechanisms "a", "mx", and "ptr" each check the records in the DNS and compare the results to the IP of the sending mailserver to see if they match. The "ip4" mechanism allows the domain holder to specify what IP address the mailserver must reside at. The "include" mechanism allows you to specify another domain's SPF data for your own domain. If you include another domain in your SPF data, then any mailserver allowed by that domain is also allowed by your domain.

Each mechanism assumes that the records specified to be checked are the ones pertaining to the domain in the sender's email address. You can specify another domain instead by adding a colon and the domain name after the mechanism. This is useful for cases where the mailservers reside in a different domain than the email address.

For example, if you include "a:example.com" in your SPF data, then when an email from your domain is received the A records for example.com are checked, and if the IP of the sending mailserver corresponds to that IP example.com resolves to, the server is valid. This works the same way for mechanisms such as "mx" or "ptr", and the "ip4" allows you to specify an IP address in place of a domain.