An mx mechanism resolves to more than 10 hosts
CODE REDWhat it means
example.net publishes N mail exchangers, and an mx mechanism may resolve at most 10. Over that. Receivers return a permanent error, a separate limit from the 10-lookup budget, and one almost nobody checks.
How to fix it
Reduce the MX records at example.net, or replace the mx mechanism with the ip4:/ip6: ranges of the servers that actually send.
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 4.6.4. 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_MX_LIMIT_EXCEEDED
Every fix on this site is yours to implement. If you would rather someone did it, I take this work directly.