I tried to configure the Mail App for my Mail Server using STARTTLS on Port 587 but it does not seem to work. The app complains that there is no connection possible whether I check to use encryption or not. In the server logs I see that the iPad connects but does not issue a STARTTLS command. I found these questions 1, 2 that suggest that iOS does not support STARTTLS but this sounds quite unreasonable. So I would like to confirm that iOS does not support STARTTLS or that the error has to be found somewhere else. Other devices can connect without problems.
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.