If you intentionally set up a mail server somewhere on your premises that automatically forwards all incoming mail to Google you may disregard this warning. Otherwise - this is a serious configuration error as it causes disruption of mail flow.
Domain should have at least 2 NS servers.
Naked domain must be an A record (not CNAME).
Every name server must reply with exactly the same TXT DKIM records.
Every name server must reply with exactly the same TXT DMARC records.
Formatting of DMARC policies.
MX lookup must fit in one UDP response packet.
TXT lookup should fit in one UDP response packet.
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