Hello Milos_Vu,
so you can open the URL
http://webmail.YOURDOMAIN.COM and see the login page? Right?
And you are using apache2, nginx and php5-fpm
( FastCGI ) ?
What happens, when you insert the right login-name
( username@YOURDOMAIN.COM ) and password? And what entries have you got in the error- and/or access - logs
AFTER you tried to login?
Are you aware, that
IF you use fail2ban and if you turned on fail2ban - filters which monitor failed logins, scripts, apache - logs, nginx - logs, mail - logs and horde/roundcube - logs, that this may lead to a temporary ban of your IP while you experiment/configure your webserver and/or if you are testing things out which are misconfigured, so you may not open the URL
http://webmail.YOURDOMAIN.COM and you may get the information, that the requested URL is not reachable? IF this is the case and you are still experimenting/configuring webserver stuff, please either whitelist your actual IP, or turn off fail2ban while you experiment/configure your webserver, to avoid being temporary banned.