When you configure your Autodiscover service, there is something called InternalURL, ExternalURL, and AutodiscoverServiceInternalURI.
You can check by running
Get-ClientAccessServer | fl AutoDiscoverServiceInternalUri, AutoDiscoverServiceExternalUri
Normally this should be something like below
AutoDiscoverServiceInternalUri : https://autodiscover.domain.com/Autodiscover/Autodiscover.xml
AutoDiscoverServiceExternalUri : https://autodiscover.domain.com/Autodiscover/Autodiscover.xml
Don’t touch the Autodiscover virtual directory for changing external and internal uri leave it as such
Actually there is no externaluri we need to specify
Internally (so when the clients are on the domain and can see the domain) they will query the domain for that value and resolve to it.
Externally the clients go through a pre-set number of URLs:
Then DNS SRV records and finally a redirect.
Therefore for Autodiscover to work correctly externally you need to have one of those URLs resolve and be on the SSL certificate – the most common method is to use Autodiscover.example.com as an additional URL on the UC certificate.
MVP – Exchange Server