In case someone wants to migrate away from Zimbra (Open Source Edition) to another system for some reason, how can all the data (i.e. mail, contacts, tasks, calendars, ...) be exported from Zimbra to some external format so that it can be imported in the new system ?

With the network edition it's rather easy I guess: just setup outlook to connect with Zimbra and export all data to a pst file.
Then setup outlook to connect with the new system and import all data from that pst file.
But this approach doesn't apply to the Open Source Edition as far as I know

Before migrating, I want to know I'm not stuck with Zimbra in case it doesn't work out for some reason (whatever that reason may be)