EWS Autodiscover Process in Hybrid with "internal" Exchange Servers

Hi Sebastian,

Have you traced the call from the middle server to the backend Server?. I'm not sure, if it is working that way. Normally you specify an "external URL" only for servers, who are reachable from the internet via that URL. This was a proper setup in the old days with exchange Servers in every location and only a central internet breakout. The central servers had an "external URL" configured but the internal Server in remote locations were not reachable. External Clients using Autodiscover would also reach the internet faced server and they calculate the AutoD Resonse based on the Mailbox Homeserver, the AD-Site, SiteLinks.
I think there is no "reverse Proxy Function" for AutoDiscover in that case. Only for MAPI/HTTP, ActiveSync, EWS etc.
Can you use https://testconnectivity.microsoft.com/tests/Ola/input  and enter your OnPrem data to check the response. ? You should see the X-FrontendServer and X-Backendserver, doing the work. You can also simulate a EXO request using https://testconnectivity.microsoft.com/tests/FreeBusy/input 

But Anyaway. If you want to control the entry Service, for a backend Server, then specify the externalURL on the Backend server to point to the frontend.

Ex2007/2010 is old but the docs are still working
InternalURL and ExternalURL settings for a non-Internet-facing Client Access server
https://learn.microsoft.com/en-us/previous-versions/office/exchange-server-2010/bb310763(v=exchg.141)?redirectedfrom=MSDN#internalurl-and-externalurl-settings-for-a-non-internet-facing-client-access-server

Could you really see "Autodiscover-Request" on the Backend Server with the mailbox from the Frontend Server? The frontend knows his own urls and can see, that the mailboxserver hat no externalURL and will us his own URL for that.