My analysis is that your client is connecting using an IMPLICITLY SECURE transport rather than STARTTLS which is what the server is expecting.
SMTP service unavailable: STARTTLS required but failed
The server you are connecting to only supports port 25 which requires STARTTLS for secure Mail.
Check if Powermta supports this or another option is to use a HOTMAIL service which support STMPS on port 465 (IMPLICITLY SECURE).
Hope the article helps you!
My testing of this service shows that there are a couple of other endpoints mentioned in the certificate
Andrei