What is it?
This mail deliverability finding means the received message did not contain a DKIM signature that could be validated. DKIM signs selected message headers with a private key, while the public key is published in DNS under a selector.
Why does it matter?
DKIM helps receivers verify that a message was authorized by the signing domain and was not modified in transit. DMARC can pass through either aligned SPF or aligned DKIM, so missing DKIM leaves less redundancy when SPF fails because of forwarding, relays, or envelope sender changes.
How to fix it?
Enable DKIM signing in the sending mail platform and publish the selector TXT record it gives you.
selector1._domainkey.example.com. 3600 IN TXT "v=DKIM1; k=rsa; p=..."After DNS propagation, send a new test message. Confirm the report shows DKIM PASS and, for DMARC enforcement, that the DKIM signing domain aligns with the visible From: domain.