Editing Sender Policy Framework (section)

===Modifiers===
The ''modifiers'' allow for future extensions to the framework. To date only the two ''modifiers'' defined in the RFC 4408 have been widely deployed:
*<code>exp=some.example.com</code> gives the name of a domain with a [[Domain Name System|DNS]] TXT record (interpreted using SPF's macro language) to get an explanation for FAIL results—typically a [[Uniform Resource Locator|URL]] which is added to the SMTP error code. This feature is rarely used.
*<code>redirect=some.example.com</code> can be used instead of the ALL-''mechanism'' to link to the policy record of another domain. This ''modifier'' is easier to understand than the somewhat similar INCLUDE-''mechanism''.