DKIM key at {selector} is below the minimum size
CODE REDWhat it means
The key at selector1 is N bits. Verifiers must not accept RSA keys under 1024 bits, so your signatures are treated as invalid, signing this mail achieves nothing at all.
How to fix it
Generate a new 2048-bit key in your mail provider and publish it, then remove the old selector once the change has propagated.
Check your own domain
This page describes the condition. To find out whether your domain has it, run the DKIM 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 DKIM, selectors and key strength, or start at the Health Library.
Where this comes from
We judge this against RFC 8301 section 3.2. Every finding on this site cites the specification behind it so you can check the work rather than take our word for it.
Other DKIM conditions
DKIM_KEY_TOO_WEAK
Every fix on this site is yours to implement. If you would rather someone did it, I take this work directly.