Automatically archive email locally

Feature summary

Companies like google mail etc. let people stock their old mails onto their servers, making it a huge database to describe customer's profile. If those people lose their account access for any reason (be it disobedience), they lose it all.

For the sake of people's autonomy, I think Geary should make it possible to move old mails on their local storage (for instance more than a month old, as does the "automatic archival" option in Evolution). That would make them take the responsibility for their data back. Let me witness this is the only feature lack retaining me from moving to Geary. I whatsoever love this program ; besides, it complies with the unix moto "do one thing, and do it well" ; compared to Evolution, the footprint on my processor usage is far more acceptable.

How I would like it to work

Obviously, the first step towards this feature will be the ability to read locally stored mails, be it MAILDIR or mbox format. I would recommend MAILDIR, because indexing of mail content works better.

Then, to make it a backup account, the more logical to me would be to make it a "ghost" account sitting beneath each IMAP account ; each mail would receive a "localCopy" flag in the MessageSearchTable, if false the message is on the server and in the MessageTable, and if true it would be on the MAILDIR storage only. There could be a logo displayed at the rightmost side of the conversation, just above the message number. In this case, each account would need a checkbox in "Storage" account prefs, called "move older mails locally", with a help button, describing where they are locally stored (dynamically checked).

And then we need to be able to search through the whole mailbox (local and serverside, which would reduce to checking MessageSearchTable for existence and location of the message).

Relevant links, screenshots, etc

Evolution displays the backup account as a separate account, which I find impractical. First it bloats the account list, and there's no option to search eg gmail INBOX through backup and server.