Please follow the below steps as givwn by Richard. It should work.
If you are not sure, speak to your IT person or share your anydesk ID and I can login to your computer and fix this for you.
We talked to Microsoft support and we had the DKIM entries in the DNS, but it was not enabled in Office365.
First check your DNS and make sure that you have two CNAME records:
selector1._domainkey Points to value: selector1-yourdomainname-com._domainkey.yourdomainname.onmicrosoft.com
selector2._domainkey Points to value: selector2-yourdomainnamecom._domainkey.LoneStarStripes.onmicrosoft.com
(replace yourdomainname with your actual domain name)
Then go into Office365.
Select Admin.
Select All admin centers
Select Security
Select Policies & rules
Select Threat policies
Select Email authentication settings
Select DKIM
Click on your domain name
click the slider from Disabled to Enable.
If this gives you a client Error it will actually give you the exact CNAMES to put into your DNS.
Select OK.
Give the DNS records some time to propagate and try turning the slider to Enable.
Once it is enabled and can see the new CNAMES it will stay on Enabled.
Select close.