DKIM Signature Verified Signature Did Not Verify

zedred

For a few days some emails have been coming back to me marked as spam. This is the indication:

X-Spam-Flag: NO
X-Spam-Score: -3.099
X-Spam-Level:
X-Spam-Status: No, score = -3.099 required = 5 tests = [BAYES_00 = -1.9,
DKIM_SIGNED = 0.1, DKIM_VALID = -0.5, DKIM_VALID_AU = -0.1,
DKIM_VALID_EF = -0.1, DMARC_PASS_NONE = -0.6, HTML_MESSAGE = 0.001,
SPF_HELO_NONE = 0.001, SPF_PASS = -0.001] autolearn = ham autolearn_force = no

I checked with https://www.mail-tester.com/ and I get a 10/10 vote.
If instead I try https://mxtoolbox.com/ by sending a test mail to ping@tools.mxtoolbox.com I get this error:

DKIM Signature Verified Signature Did Not Verify

Any idea?