Which mail file to keep before a completely fresh install?

["P.J. Berg"]

Hi.

In my current mail directory there is quite a bit I doubth should be there  
from a previous upgrade attempt from 9.27 to 9.50.

I want to keep all my e-mail, other settings are not vital.
In the current mail directory(repeating myself) there are two mailbase  
files, named: Mailbase.dat and omailbase.dat

It is easy to see which is the larger, so the question is: Do I need to  
keep both?
Is there any other files I need to keep to be able to restore my e-mail?

And why is there no export function for e-mail in Opera?

Sincerely.
J.Berg

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