External DNS records required for Office 365 (core services)

Every Office 365 customer needs to add two records to their external DNS. The first CNAME record ensures that Office 365 can direct workstations to authenticate with the appropriate identity platform.

The second required record is to prove you own your domain name.

TXT
(Domain verification)

Used by Office 365 to verify only that you own your domain. It doesn't affect anything else.

Value:

Host: @ (or, for some DNS hosting providers, your domain name)
TXT Value: A text string provided by Office 365
The Office 365 domain setup wizard provides the values that you use to create this record.