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When you set up Office 365, DNS records are required so that email, instant messaging, and other services work correctly. Depending on the Office 365 subscription you signed up for (Enterprise, Midsize Business, or Small Business), you might have a choice between managing your own DNS records at your DNS hosting provider (called partial redelegation), and having Office 365 create and manage the DNS records for your domain (called full redelegation).
You can read more here: