Hi,
I didn't solve this problem altgouh investigate in forum. :(
I applied these steps, https://www.digitalocean.com/community/tutorials/how-to-install-your-own-webmail-client-with-roundcube-on-ubuntu-16-04 (https://www.digitalocean.com/community/tutorials/how-to-install-your-own-webmail-client-with-roundcube-on-ubuntu-16-04)
But I still take this error when first login the system.
$config['default_host'] = 'ssl://imap.domain.com';
// TCP port used for IMAP connections
$config['default_port'] = 993;
$config['smtp_server'] = 'ssl://smtp.domain.com';
$config['smtp_port'] = 465;
This is discussed quite a lot on this forum. First thing to check is if you are having a certificate verification problem. Please see https://github.com/roundcube/roundcubemail/wiki/FAQ#problems-connecting-imapsmtp-server-via-ssltls