Hi,
First, we need to create DKIM, DMARC in local DNS zone, each one should be created as a TXT record and then you add the custom domains and IP addresses.
After that, publish them in public DNS.
For detailed information about the TXT records and their implement process, you could refer to the following documents:
SENDERID, SPF, DKIM AND DMARC IN EXCHANGE 2016 – PART I
SENDERID, SPF, DKIM AND DMARC IN EXCHANGE 2016 – PART II
SENDERID, SPF, DKIM AND DMARC IN EXCHANGE 2016 – PART III
(Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)
Regards,
Perry
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