I think that with ipv6 listening on port 143, the situation is the same. So in simple words, your server is listening.
Keep in mind that
1) You must have a working mail (e.g.
the@maildomain.com -- create it through panel)
2) Your login name must be that one (the@maildomain.com --enter it in the user name field of roundcube)
3) The roundcube and courier must be correctly installed.
If you are not sure about roundcube rerun
Code:
dpkg-reconfigure roundcube-core
For courier follow the perfect server