Hi @Namless Shelter , in a Exchange 2010 and 2016 coexistence environment, we will make sure the DNS record point to the Exchange 2016 server, and set AutoDiscoverServiceInternalURI: https://autodiscover.domain.com/Autodiscover/Autodiscover.xml for both Exchange 2010 and 2016 server.
For the mailboxes on Exchange 2010 server, the requests will be proxyed by 2016 to them.
Internal DNS
webmail.domain.com Point to Exchange 2016 server
autodiscover.domain.com Point to Exchange 2016 server
External DNS
webmail.domain.com Point to Exchange 2016 server
autodiscover.domain.com Point to Exchange 2016 server
Referring to this link to get more information: Client Connectivity in an Exchange 2016 Coexistence Environment with Exchange 2010