POP? It's leaner.

Seriously, it's an absolutely perfect match for exporting everything, and it's easy to set up, and you get it in the format of your choice. Infinitely easier than having them support only formats they have time to write exporters for. It's also easily resumable, unlike the 8GB zip file I'd otherwise have to download.

You do realize that POP only supports your Inbox, right? The protocol has no concept of 'folders' or 'tags.'

I think if you switch it to "all mail", it downloads everything. Though you might be right.

POP might be a start, though for a proper backup tags need to be included somehow, maybe in a custom header field.