I have Postfix 2.7 on my Debian Lenny mail server with TLS support and this option in config:
smtpd_tls_auth_only = yes
Description (http://www.postfix.org/postconf.5.html#smtpd_tls_auth_only)
And I think because of it Roundcube can't send messages throw my SMTP. Test in installer say
Trying to send email...
SMTP send: NOT OK(Authentication failure: SMTP server does not support authentication (Code: ))
So, I think, Roundcube try to authenticate before start encrypted connection, but in this case my server say, that it doesn't support authentication.
So Roundcube should do all checks in correct order: at first start TLS connection if server support it, and only after that try to check authorization.
Hm, sorry, I've change my SMTP hostname to tls://localhost and all now work perfectly!