These issues might be familiar to experienced administrators.
The external host name you choose should ideally be one that is already included in the Exchange certificate configured on the Client Access server.
There are three main tasks to deploy Outlook Anywhere in an Exchange environment: Client Access, and select the Client Access server you want to enable for Outlook Anywhere.
If you have multiple Client Access servers in an Active Directory site then choose the one that is the internet-facing Client Access server.
Previously this was known as RPC-over-HTTPS but was renamed to Outlook Anywhere in Exchange 20.
By wrapping normal Outlook RPC requests in HTTPS the connections are able to traverse firewalls over the common SSL/HTTPS port without requiring the RPC ports to be opened.