This isn't a Roundcube problem, something is wrong with your Dovecot configuration or possibly ISPConfig isn't setting up the account correctly.
True, I fixed it for anyone who wants to know how:
(I am using CentOS 7 so it may be different for you)
1 ) Go to /etc/postfix
2 ) Find the main.cf file
( or just do nano /etc/postfix/main.cf )
3 ) and edit it. ctrl+f for the following:
mydestination - remove your domain from it and make sure it looks like this:
mydestination = localhost, localhost.localdomain
search for myhostname
and make sure that the domain that shows up is NOT your emailing domain.
Ex: if your email is alex@emailservice.com
and myhostname is:
myhostname = emailservice.com you have to change it to something else. for example you have to change it to server.emailservice.com
and make sure "mydomain" is either commented out or is NOT your emails domain.
and then restart postfix.