I´ve found this file:
:/var/run/apache2# tail roundcube-sqlite.db
uid integer NOT NULL default '0',
subject varchar(255) NOT NULL default '',
"from" varchar(255) NOT NULL default '',
"to" varchar(255) NOT NULL default '',
"cc" varchar(255) NOT NULL default '',
"date" datetime NOT NULL default '0000-00-00 00:00:00',
size integer NOT NULL default '0',
headers text NOT NULL,
structure text
øÞ';O^Þ01cc64f52b593046c9c3200eb75bb8012010-02-20
Where to see wether the ID is used or not!?!