The following is the environment:
One Exchange 2013 and one 2016 server. The 2016 server is in hybrid mode. None of the the client access URLs point to the 2016 server FQDN. Autodiscover.domain.com (split DNS) points to autodiscover.outlook.com as all mailboxes (except journaling and some test mailboxes) are in Office 365.
The official cert is installed on Exchange.
Some Outlook clients get a certificate warning and it shows that they are trying to connect to the local Exchange 2016 server's FQDN - which gives the error of course because this name is not on the certificate.
Any idea how Outlook keeps connecting to this while none of the CAS URLs point to this FQDN and while autodiscover points to Exchange Online?