Just to clarify the discussion of OfflineIMAP above, it won't work with Modest as an email client, you need to use Mutt in an xterm or some other mail program that supports Maildir as a local cache.

That's a deal-breaker for me personally, I can't live without a finger-friendly UI. So at the moment I'm playing an absurd game where when I want to cache email, I install my version of libtinymail-camel, then when I'm done I install the official libtinymail-camel to read them. It's good enough for the moment.

I submitted this bug to tinymail:
http://tinymail.org/trac/tinymail/ticket/91