Thanks ZhangHuangbin I open "/usr/share/apache2/roundcubemail/config/db.inc.php" and the only variable I found that could be useful is "$rcmail_config['db_dsnw'] = "mysqli://" I change it to "$rcmail_config['db_dsnw'] = "mysqli:roundcube:VUzgoomtJBhwAxxxxxxMvOP3KiC@127.0.0.1/roundcubemail";" and create the user roundcube with password VU*iC  in mysql, but still not working.

I check the user roundcube with the password I assigned to it and it seams to work.
I have a series if warnings in /var/log/mail.warn similar to:

Apr  2 19:15:40 server2 postfix/pickup[27724]: warning: 9186B39E014F: message has been queued for 79 days
Apr  2 19:15:40 server2 postfix/cleanup[28174]: warning: proxy:mysql:/etc/postfix/mysql/sender_bcc_maps_user.cf lookup error for "root@{mysite.com}"
Apr  2 19:15:40 server2 postfix/cleanup[28174]: warning: 9186B39E014F: sender_bcc_maps lookup problem
Apr  2 19:15:40 server2 postfix/pickup[27724]: warning: maildrop/EEF5B39E489C: error writing 9186B39E014F: queue file write error

After checking the log I opened the file "/etc/postfix/mysql/sender_bcc_maps_user.cf" it had a username and password not recognize by the mysql, the username is "vmail" I created the user and tested manually, the warnings seem to stop, but the mail still not working. I want to point out that it used to work, the story goes like this I installed iremail in a fresh server with one domain, I tested and it work just perfect, the I added more domains and tested and work just fine, then I added a last domain which had conflict with my server name and even then the mail seems to work, finally I change the server name and since the mail stop working. the conflict was that the server name was the same a one domain and the server was redirecting all domains to the one of the server name.

It should be "mysqli://", you missed '//' here.

Please make sure you have correct SQL username/password in config files.