IIS 10 SMTP Server certificate error

Hi Help please!

I am trying to set up IIS SMTP Server in Windows Server 2019. I have done similar in Server 2016 with no problem.

Any attempt to send email fails with the error message Exception Message: Exception calling "Send" with "1" argument(s): "The remote certificate is invalid according to the validation procedure."

The certificate is from GoDaddy and is valid. The system recognises that the certifcate is present in Event log message from smtpsvc: "A TLS server certificate was found for SMTP virtual server instance '2' with thumbprint 89b3bab82xxxx..... TLS will be available for this virtual-server."

The SMTP Server is set up as a Relay to Office 365 using TLS. The FQDN on the Advanced Delivery tab of virtual server properties matches the Common name of the Certificate.

Any help much appreciated

Thanks