If you choose to keep your %BRAND_EXCHANGE_ONLINE_SHORT% Server instead of decommissioning it, you should also make sure ...

If you choose to keep your %BRAND_EXCHANGE_ONLINE_SHORT% Server instead of decommissioning it, you should also make sure that the Autodiscover DNS CNAME record points to %BRAND_OFFICE_365_SHORT% in both internal and external DNS after the migration so that the %BRAND_OUTLOOK_SHORT% client will connect to the correct mailbox.