How do I convert outlook pst files in mbox format?

[Peter K.]

Over the years, I accumulated a significant number of .pst files (from 50 to
500Mb, each with a more or less complex folder and sub-folder structure).

How can I convert these in a more standardized mail archive format (eg.
mbox?), accessible by other mail programs, e.g. Entourage or Mail.app for
Apple Mac, Thunderbird, ...
Or, are there any other programs (except MS Outlook), that can read .pst
files?

Thanx.

Peter