Sender Policy Framework

Some SPF implementations predate the final version of RFC 4408 and the standards track update, RFC 7208, see the Implementations page for details. While many of the changes between the pre-IETF specifications and both the final experimental and standards track RFCs are minor or cosmetic, the processing limits are very different. These were further modified in RFC 7208 Section 4.6.4. These processing limits were adopted to mitigate the risk of various types of denial of service attacks to which the pre-IETF protocol and programs were potentially susceptible.

If you are not using a program that supports the RFC 4408 processing limits, you should start planning to migrate. If you don't know ask your vendor. The Implementations page gives information on the compliance status as it is known to the project.