Hi Herman,
Problem solved! I don't know the reason but testing with the Send Test Email button in the ClearPass Policy Manager > Administration » External Servers » Messaging Setup section didn't work. Then I have tried with Send Test Message button in the ClearPass Guest > Home » Configuration » Receipts » Email Receipt section and worked, I received the test email in my Gmail inbox:
BTW, I have tested this successfully with the three security options (none, SSL, StartTLS) and I didn't mind about the trust list as you did.
I don't know why I get that error with the Send Test Email button in the ClearPass Policy Manager > Administration » External Servers » Messaging Setup section.
For your last question, I only have configured the management interface.
Now I will see if it works with a real message...
Regards,
Julián