[DKIM] Your DKIM signature is missing.
The message was not signed. This means that the message had no DKIM signature. This is not the same as failing.
What is the cause of this error?
I can say, I host the image file as my email signature as a domain that does not match my email's domain.
Is this the problem?
On a related note, what is the SPAM impact of hosting my email signature image on a domain that does not match my email's domain?