Is there a full-featured Mac email app that natively implements labels (instead of folders) and archiving? I love Gmail's features, but I'm increasingly wary of having my entire email history depend on Google. Every Mac email app I know of, other than Sparrow, insists on being based around IMAP folders. Different clients integrate with varying levels of smoothness with Gmail, but at their cores they still use the file-it-or-delete-it paradigm.

Sparrow, of course, was designed as a lightweight Gmail client. It understands Gmail's labels, and it understands archiving, but it depends on the Gmail web interface for lots of core email features (filters, etc.), and I don't know how much of its Gmail-ness is lost if you use it with regular IMAP accounts.

What I want in an email app is this:


Native Mac app
Implements labels itself for all supported account types, not just Gmail
Allows archiving as the default "I'm done with this" action, even for unlabeled mail
Handles any discrepancy between these features and the underlying email protocol (IMAP, POP, whatever) under the covers with as little leakage into the interface as possible
Has all the usual power features that exist in other apps like Mail and Postbox
For bonus points, stores email on disk in some format that isn't a complete binary black hole


In general, I want my email to act like Gmail but live on my computer; and no matter what happens on any server anywhere, as long as I have a backup of this app's archive files, I should be able to move them to another computer, install this app on it, and have 100% continuity with zero fidelity loss. It's okay if the app DOES use Gmail as a back-end, as long as I don't lose any data fidelity if Gmail's entire database disappears.

(Making such a setup work with mobile devices is a whole other problem, but I'll gladly deal with that tomorrow for this app today.)

So: I think what I've described is sufficiently difficult (dare I say "unfeasible"?) that it doesn't exist yet. I would love to find out that I'm wrong. Am I? If not, what's the closest thing to this setup that can be had on the Macintosh?
I have two backup recommendations. Although these are not apps you'd use to manage your mail, they do backup your account to your system so you can use whatever management client you like (e.g. Sparrow, Mail, etc.). 

I use getmail by Charles Cazabon to back up my entire Gmail account silently in the background. The emails are backed up into a large .mbox file, accessible by any number of mail clients. This also makes it easy to move your entire account from one machine to another, or to restore. The program says it works with any IMAP account - support folders, labels, etc. I only use it with Gmail.

A second backup option is Gmvault. There's a Windows GUI installer but for OS X one still needs to go to the command-line. Again it backs up your Gmail account (the sync option) and stores the emails locally on the file system. Emails are stored individually as .eml with an accompanying metadata file. You can manipulate those files as you like without messing up the gmvault functionality. There's also a restore option, and it's suppose to handle IMAP discrepancies with Gmail well, although I've never encountered any problems there that (of which I'm aware).