According to this forum post on Microsoft Community, many people are complaining that STARTTLS isn't working for them either. A Microsoft representative claims on page 3 that the Mail app will exhibit the following behavior in relation to STARTTLS:
If Server support STARTTLS, Mail app will do unsecured connection
follow with TLS handshake.
If Server doesn’t support STARTTLS, Mail
app will send email with unsecured connection.
Later he states:
FYI, mail app always connect server with secured connection whenever
it is possible (STARTTLS capability was represented in this case).
We are looking to patch this issue with fallbacking to unsecured
connection if TLS connection failed.
Based on that, it seems like it's possible to use STARTTLS, but many others are claiming they cannot get it to work either.