After migrating all email accounts from an on-premises Exchange 2008 or later server to Exchange Online, there remains the problem of what to do about new Outlook profile creation. Outlook will still look for the old server name, and especially if you want to keep the old server alive for a while, you will have significant problems getting Outlook 2013 to do anything with Exchange Online. Here is what the extraordinary Matt Quick and I did recently with beautiful results.

For the sake of this discussion, “localdomain.local” is the LAN-local AD-enabled domain, and “publicdomain.pub” is the Internet domain. The on-prem Exchange originally had local DNS name “exchange.localdomain.local” and Internet DNS name “exchange.publicdomain.pub”.

1. Migrated all mailboxes from on-premises Exchange 2010 to Exchange Online. Dirsync was used for initial account setup, then turned off for the actual copyover process which was done with MessageOps.
2. In Exchange Management Shell, ran Get-ClientAccessServer to get the canonical name of on-prem Exchange (we’ll say it was EXCHANGENAME), and then Set-ClientAccessServer -Identity EXCHANGENAME -AutoDiscoverServiceInternalUri $NULL (replacing EXCHANGENAME with the actual name) to nullify as many defaults as possible.
3. Set autodiscover.localdomain.local as a CNAME to autodiscover.outlook.com.
4. Set autodiscover.publicdomain.pub as a CNAME to autodiscover.outlook.com in Internet DNS. This LAN has a local copy of publicdomain.pub in its domain controllers, so copied this record to the local server as well.
5. Unregistered the NIC for the on-premises Exchange server in DNS. The checkboxes are in the DNS tabs of both TCP/IPv4 and TCP/IPv6, within the Advanced area of the NIC. This is done so that DNS changes which are next, will not be undone automatically.
6. Removed DNS A records exchange.localdomain.local and exchange.publicdomain.pub from local and Internet DNS respectively.
7. Added DNS CNAME records exchange.localdomain.local and exchange.publicdomain.pub, both pointing to outlook.office365.com, to local and Internet DNS as appropriate.
8. Set up oldexchange.localdomain.local and oldexchange.publicdomain.pub as A records pointing to the IP being used by the on-premises Exchange, to local and Internet DNS as appropriate, for archival uses and until we are ready to decommission the on-prem Exchange altogether.