it's clear in the log:
Jun 23 22:06:15 vpl postfix/trivial-rewrite[2461]: warning: do not list domain vpl.si in BOTH mydestination and virtual_mailbox_domains
This is WARNED in iRedMail installation wizard, your virtual mail domain name cannot be the same as your server hostname (vpl.si).
Solution:
*) Set your server hostname to another one. for example, mail.vpl.si, mx.vpl.si, etc. A server reboot is required on some Linux distributions to use the new server hostname.
*) Update Postfix config file /etc/postfix/main.cf, set "myhostname =" to the new server hostname.
*) Restart Postfix service.