I would just go through the settings to make the changes for the SSL certificate.
As already stated, the best practise is now considered to be that both the internal and external URLs are the same, with the external URL being used via a split DNS system.
It could be that you have enabled the wrong certificate or the wrong web site. I have seen HTTPS get bound to the backend site (which uses a self signed certificate) in the past, so you should check that. Check through EMS and get-exchangecertificate that your trusted certificate is still bound to IIS.
Simon.
Will Szymkowski
As stated your internal and external URL's for your virtual directories should be the same. Did you renew this certificate and re-import it into the CAS server? If you did then you will also need to use the Enable-ExchangeCertificate -Thumbprint xxxxxxxx -services "pop,imap,smtp,iis" command to ensure that you are using this cert for Exchange. This command also needs to be run on all of your CAS servers that hold the cert.
Also a good test is, on the client that is getting the error message, Open Outlook, chold ctrl+right click the Outlook icon in the system tray and select , Test Email Auto Configuration run the Autodiscover test and check the Results Tab to ensure that the virtual directories are correct.
As already stated, the best practise is now considered to be that both the internal and external URLs are the same, with the external URL being used via a split DNS system.
It could be that you have enabled the wrong certificate or the wrong web site. I have seen HTTPS get bound to the backend site (which uses a self signed certificate) in the past, so you should check that. Check through EMS and get-exchangecertificate that your trusted certificate is still bound to IIS.
Simon.