Email systems use a DNS record called an MX record to figure out where to deliver emails. During the email migration process, your MX record was pointing to your source email system. Now that the email migration to Office 365 is complete, it’s time to point your MX record at Office 365. This helps make sure that email is delivered to your Office 365 mailboxes. Moving the MX record will also let you turn off your old email system when you’re ready.
This will stop incremental sync
Note: In Exchange 2007, Exchange 2010, and Exchange 2013 you should also set Set-ClientAccessServer AutodiscoverInternalConnectionURI to Null.
Office 365 uses a CNAME record to implement the Autodiscover service for Outlook and mobile clients. The Autodiscover CNAME record must contain the following information:
Should you have one, click here