Click on Admin > Show All > Exchange > Protection > DKIM (Top Nav. Menu)
Yourdomain.com > Authoritative > Enable
Unfortunately, Microsoft has changed the procedure for this, and now in some cases they will require the user to enable DKIM from the previous step using their PowerShell. The PowerShell only works with a PC or Windows computer. I have a Macintosh and consequently couldn’t enable my DKIM using PowerShell. To get help with PowerShell, we recommend contacting Office365 support – bottom right corner of your Office365 Admin Dashboard.
If you're lucky, you might have a domain that doesn’t require PowerShell to enable the DKIM DNS records. But It’s impossible to determine which ones will allow an easy enable versus PowerShell enable. It’s important to note that the DNS propagation into Office365 can take up to 72 hours.
How to enable DKIM easily:
Click on Show, All on the left-hand side navigation menu
Click on Exchange.
Click on Protection on the left-hand side navigation menu
Click on dkim on the TOP navigation menu.
Click on your domains and enable DKIM.
You will see two domains: your actual domain and the Microsoft domain.
DMARC - Domain-based Message Authentication, Reporting, and Conformance
Go to your domain admin’s site and open the DNS manager
Create a TXT Record
Time to Live (TTL) = Leave at the default or enter 3600 or 1 hour
HOST NAME: _dmarc
VALUE (with email): v=DMARC1; p=none; rua=mailto:firstname.lastname@example.org
The email version will send reports to whatever email you put in there.