Exchange 2019 ssl certificate invalid

belilan,

 From your picture, it looks like your CSR might not have been correct, or your certificate wasn't correctly installed, as I do not see your Exchange Virtual Directory URL under the name (Figure A).

I understand the steps you have taken; you purchase a domain; let say for this example, it is contoso.com, and your exchange server virtual directory URLs are exchange.contoso.com. You created a Certificate Sign Request (CSR) for exchange.contoso.com and submitted that request to a certificate authority. Once you received your certificate, your complete a pending certificate request. Which then, you are assign services to that certificate.

Figure A:

The following posting show how to installed and configured certificates. https://docs.microsoft.com/en-us/exchange/architecture/client-access/certificate-procedures?view=exchserver-2019

Reference: https://supertekboy.com/2016/08/02/renew-a-certificate-in-exchange-2016/