SPF mechanism uses = instead of :
CODE REDWhat it means
the offending value is written with an equals sign, which makes it a modifier rather than the mechanism you intended. Receivers ignore unknown modifiers, so those senders are silently unauthorised.
How to fix it
Mechanisms take a colon: write include:example.com, not include=example.com. Only redirect= and exp= use an equals sign.
Check your own domain
This page describes the condition. To find out whether your domain has it, run the SPF test or the full checkup, which examines every record at once.
Understand it properly
This page covers one condition. For the whole picture, What is SPF, and the ten-lookup limit, or start at the Health Library.
Where this comes from
We judge this against RFC 7208 section 12. Every finding on this site cites the specification behind it so you can check the work rather than take our word for it.
Other SPF conditions
SPF_INVALID_TERM_SEPARATOR
Every fix on this site is yours to implement. If you would rather someone did it, I take this work directly.