I have been moving some users from one server to another when one complained that his address book had gone missing. For a moment I got a bit paranoid then I reasoned the information must exist somewhere.
I copied all of his email but somehow the addresses were gone. After much searching I found that he used Squirrel Mail, which stores addresses and preferences in a data directory. In Ispcp installations these files are per user and stored in /var/www/ispcp/gui/tools/webmail/data Each mail user (who has logged into webmail) will have a user.abook and a user.pref and copied his to the new server and his addresses were restored.