Microsoft Exchange couldn't find a certificate that contains the domain name mail.smtp25.org in the personal store on the local computer. Therefore, it is unable to support the STARTTLS SMTP verb for the connector HTS-IronPort with a FQDN parameter of mail.smtp25.gov. If the connector's FQDN is not specified, the computer's FQDN is used. Verify the connector configuration and the installed certificates to make sure that there is a certificate with a domain name for that FQDN. If this certificate exists, run Enable-ExchangeCertificate -Services SMTP to make sure that the Microsoft Exchange Transport service has access to the certificate key.
Get-ExchangeCertificate | FL
You will need to highlight the Thumbprint and paste into fallowing PS command
Enable-ExchangeCertificate –Thumbprint DBA3601A10DF90E0F00462C03940C90B8BA87292 -Services "SMTP"
oz Casey Dedeal,
MCITP (EMA), MCITP (SA)
MCSE 2003, M+, S+, MCDST
Security+, Project +, Server +