In the lab I have completed an Exchange 2010 - 2016 migration ahead of doing it live in production. I setup split DNS, configured Exchange 2016, changed the DNS records to point to the new server for mail and autodiscover and I am at the point of migrating mailboxes.Using Outlook 2016, if the mailbox is on the Exchange 2010 Server, it works. If I run a Test email configuration it shows that autodiscover is on the old server. Shouldnt this show the new server? If I migrate the mailbox over to 2016, Outlook 2016 wont connect to the Exchange server and if i run the same autoconfig test, autodiscover fails. If I try to create a new mail profile, it will crash Outlook. A brand new user with new mailbox on Exchange 2016 works fine, no issues. I have migrated the arbitration mailboxes. I can ping autodiscover and it resolves to the correct IP address / NSlookup resolves correctly as well. Both basic and NTLM authentication is set on the autodiscover Virtual Directory. OWA works fine.