Hello,
> I have 25400 mails in my archive mailbox. :) In fact, I did the
> opposite of what you did: joining different archive mailboxes.
> The goal is to make searching easier.
have a look at mairix.
> > Can I convert my mboxes to maildir?
> Yes, with Mutt.
I used mutt, too. After I fixed several bugs in other softwares which
should be specialized in converting mailboxes.
Thomas