I'm surprised there aren't other threads about this in Google searches, and I'm seemingly the first to ask this. Anyway:

There are apparently a few advantages to having Akonadi access your emails. Such as Desktop Search being able to find them, or the Lancelot launcher showing unread emails in the Contacts section. I assume this would automatically work if I used KMail, but I use Thunderbird as my email client and do not intend to change that. Now it does seem I could simply create an "Imap / POP3 E-Mail Server" in Akonadi... but since Thunderbird fetches my emails already, why use twice the bandwidth and possibly disk storage?

So is there a way to integrate Akinadi with Thunderbird, and make Akonadi able to see emails in a local TB folder? There doesn't seem to be a Thunderbird plugin for Akonadi by default, so I imagine it might be tricky. I use standard email storage in TB, and my Y! Mail path is ~/.thunderbird/0s6dl970.default/ImapMail/imap.mail.yahoo.com/ Could Akonadi be tricked into reading mails from there?

If not, the question can be asked the other way around too; Can Thunderbird use the location and format of a KDE local folder? I imagine that would also make Kmail directly able to work with it, in case I ever have to pause on Thunderbird for some reason. This would also be a preferred solution if possible.

Just tried something out which nearly worked. I added a MBox resource in Akonadi, and pointed it to my Thunderbird INBOX file which is actually mbox storage. The path was /home/mircea/.thunderbird/0s6dl970.default/ImapMail/imap.mail.yahoo.com/INBOX. Problem is that it makes the akonadi process use 13% CPU for a few minutes then consume 1.6 GB of memory permanently, which means this is not an option. It did however allow KMail to see the Inbox directory from Thunderbird. Let me know if there's any alternatives which can work around that, or something else entirely.

I found my own solution to this problem. Maybe not the best one, but one that works very well still.

First, you must make Thunderbird use maildir storage rather than mbox, which should be better and faster either way. It's a hidden setting, and to do so you must open about:config and set mail.serverDefaultStoreContractID to “@mozilla.org/msgstore/maildirstore;1″ then re-configure all email accounts.

Once that's done, you simply add a new Maildir resource in Akonadi, and preferably set it as read-only so there's no risk of interference. Specify the path to one of Thunderbird's per-account local folders... in my case I now moved my Gmail to "/home/mircea/.local/share/imap-mail/imap.googlemail.com" for example. Akonadi will automatically see all emails in my local Thunderbird folders and make them available in KMail too as an added bonus.

Note that this doesn't mean you can get / send email from KMail automatically, since the POP3 / IMAP settings and actual folder interpretations are Thunderbird specific, and KMail would need its own (such as an IMAP Akonadi resource). But Akonadi seeing my Thunderbird emails is all I wanted, so I don't care about that.