Today I was testing Outlook with a new Exchange server config and I received the following error :
I knew I correctly imported the new SSL certifcate because I tested OWA before Outlook and that was working fine. The solution was simple. You must specify in Exchange which Exchange services (for example SMTP, IIS, POP, UM etc) must use which certificate.
Open the Exchange console :
Now go to the right pane, select the correct certifcate and choose
Assign Services to certifcate. Make sure you select IIS and SMTP for OWA and Outlook functionality. In my case the Outlook error disappeared.