How to import users mailbox from Plesk 8.1 Horde

Hello all,

I'm moving servers and need to migrate my domain users mailboxes currently in Plesk 8.1.1 Horde mail to Squirrelmail. I run a Debian server now with ISPCP.

On the old server (Plesk) I have made a backup of the domain moved the backup file to my new server extracted all the data. What's the next step to import the users mail data to Squirrelmail?

Thanks,

Rob