I was unable to send Mail from my N9, but thanks to the detailed explanations on the wiki the issue was easy to debug. I'm just posting this here in case anyone has a similar issue and to say thanks for the great help on the wiki and in this forum.

Our mail server is configured to accept plain password authentication over TLS; however, only AUTH PLAIN was supported. Even though the mailserver reported this correctly, Mail on the N9 always tried to authenticate with AUTH LOGIN.

I don't know if this is documented anywhere, but in case you're unable to send mail with a similar configuration, check whether or not your server supports AUTH LOGIN. After I enabled AUTH LOGIN on the server everything works perfectly.