For converting emails into archival formats, the two open source tools I'd look into are ePadd and Mailbagit.
If at least some subset of these emails are things you'd like to explore making searchable/viewable by the general public, I run fwdeveryone.com and am happy to chat.
Thanks. Will check out both ePadd and Mailbagit.
Unfortunately, there's no really publicly accessible content in these mails. At least none that I can classify in any meaningful way through a filter or search criteria.
I was looking at mbox ... would you be able to recommend a simple (ideally, read-only) client which we can use to open and browse/search the mails in a mbox, without importing the content into a regular mail client.
Mbox is text, so it is searchable with something which takes a huge file. Not sure you will find client type formatting as standalone option. I use Thunderbird which is at least local.