Hi,
we have the same setup. Postfix + Mysql, Amavis (avira (primary), clamav (backup), spamassasin), courier-pop(-ssl), courier-imap(-ssl), roundcube.
In your setup the Imap/POP-Server is missing.
If you have a working Imap-Server Roundcube can access your mailserver without any problems.
I think google is your friend: postfix +mysql +imap
Greetings, Jesko