>> This is very strange and I can confirm it. > > Can you test the other (working) certificate again? In Outlook set the > hostname as per certificate and in local hosts file in Windows force > IP of the destination server for this hostname. This way Outlook > should not complain about mismatched hostnames...
Unfortunately I can't experiment with certificates (customer says this has been enough trouble). Can you check the certificates' serial numbers? The working one begins with 03 and the problematic one with 04. There are 37 archived certificates for this hostname, 29 begin with "03" and only 8 with "04". Certificates starting with "04" occur since autumn 2019. After that date it's sometimes "03" and sometimes "04". Best regards Gerald
