Generally this condition occurs if anyone of the following conditions is true:
1. The fully qualified domain name (FQDN) that is specified in the Warning event has been defined on a Receive connector .
2. Send connector on a Microsoft Exchange Server 2007 transport server, and no certificate is installed on the same computer that contains the FQDN in the Subject or Subject Alternative Name fields.
3. A third-party or custom certificate has been installed on the server and it contains a matching FQDN. However, the certificate is not enabled for the SMTP service.
The warning event indicates that there is a problem loading a certificate to be used for STARTTLS purposes, here is how to resolve these issues.
1. Open “Exchange Management Shell”.
2. Write “get-ExchangeCertificate” and press on “Enter” button.
3. Write down the Thumbprint of the certificate that reflect the required FQDN name of the server.
4. Review the current certificate that use by the Exchange server and
each certificate function.
5. Write “Enable-ExchangeCertificate -Thumbprint ThisIsYourThumbPrintKey -Services “SMTP”
and press on ‘Enter” button. Replace “ThisIsYourThumb…” with the value of -Thumbprint obtained in stage 3.
6. Restart the Exchange server.
Now your cooking with SSL Security!!.
For Creating a Certificate or Certificate Request for TLS check the below article