The mailman mailing list manager allows to download monthly archives in "Gzip'd Text" format. This format is not mbox but can easily be turned into it by the following simple line of sed:
sed 's/^\(From:\? .*\) \(at\|en\) /\1@/'
This makes it much easier to browse paste emails of an archive eg. using mutt or reply to past emails with a proper Message-ID as to not break threads.