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. email@example.com
-- create it through panel)
2) Your login name must be that one (firstname.lastname@example.org --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
For courier follow the perfect server