Bulk export of emails

Continuing the discussion from Export email (.eml):

Is there a way to export ALL emails from a specific timespan to eml-files?
Maybe via API-calls or so?

A customer wants to have these emails in a separate Mailstore and asks for a way of doing that. The mails were fetched from an Exchange server and deleted after fetching … so they don’t have the mails on the Exchange server anymore.

thanks for any pointers, Stefan