SMTP with STARTTLS on iPad
The problem was not STARTTLS, I was able to verify that the mail app supports STARTTLS. My problem is that the iOS 9 mail app does only support old and insecure SSL/TLS protocol versions. Usage of these versions (TLSv1.0, TLSv1.1, SSLv2 and SSLv3) is forbidden by my mail server for security reasons. Since the iOS 9 mail app only supports these insecure protocol versions no connection is possible.

See also 1 and 2.