Hey Peter,
Thanks for the question. Please go to Settings->Bindings->Ports. After this select port 25 and set the encryption type to Start TLS. After this select the certificate that you want to use. If you have multiple certificates in the certificate folder it will use SNI. After doing this please try the test again.
Thanks,
Zach Sylvester
Software Developer
SmarterTools Inc.
www.smartertools.com