DMARC record not found
No DMARC TXT record was found at _dmarc for this domain.
Why it matters
Without DMARC, receivers have no published policy for mail that fails SPF or DKIM alignment.
What to do
Publish a DMARC record. Start with p=none and rua= if you need reports, then move to quarantine or reject.
Reference
DMARC publishes policy for handling messages that fail SPF/DKIM alignment.