T_DKIM_INVALID DKIM-Signature header exists but is not valid

I have set up DKIM on our corporate mail hosted by GMail. Google
assigned a TXT record and our DNS-provider set it.

I send a mail from GMail from the Web UI and from my Thunderbird to
myself, and SA always triggers that rule.

What does it mean? Is not valid?

-- There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy. -- Wm. Shakespeare, "Hamlet"

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