So you've imported your Certificate and you want Exchange to use it. The perfect job for *dun da da daaa* the Enable-ExchangeCertificate commandlent
Here's how it works
Enable-ExchangeCertificate -Thumbprint <String> -Services <None | IMAP | POP | UM | IIS | SMTP> [-Confirm [<SwitchParameter>]] [-DomainController <Fqdn>] [-Force <SwitchParameter>] [-WhatIf [<SwitchParameter>]]