[mew-int 2730] format used by mew for local messages

hello again,

I would like to use Mew to download all my gmail messages (via IMAP), 
so I can search them using Hyper Estraier.  But before I do this (I 
think it will be quite involved (1)), I would like to know more about 
the format which mew uses for downloaded copies of messages.  Is it 
correct that other e-mail programs do not recognize it?  Can I make 
Mew save the messages from an IMAP folder as mbox or maildir?

Using a format exclusive to Mew is fine to me, but maybe I will no 
longer be able to use Mew in the future, and it would be unpractical 
if I could no longer read my e-mail archive.

Are there any solutions?

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perhaps not strictly on-topic for Mew-int:
(1) Any hints on how to go about downloading all these messages are 
also welcome.  I have about 2,8 Gb of messages there. I think I will 
have to divide them into multiple folders (perhaps per month or so) 
and download those folders one by one, so that Mew can handle them?
====

thank you!

Thomas