If you like to keep old e-mails, you've got quite a few ways you can go about storing them. Sure, leaving messages in the Mail application (or even stuffed in your inbox) for the rest of time is one choice. Just because that would drive me batty doesn’t mean it’s not a viable solution for you.
A different (and in my opinion, better) option is to take old messages out of Mail altogether. I mean, if it's possible you'll need to access them again, it's good to have an archive, right? But it's not entirely necessary for you to keep them cluttering up Mail if the information is years old. So you could export your mailboxes out in what’s called mbox format, ready to reimport the data if need be.
However, there are a couple of caveats to archiving your mail that you should be aware of. First, know that this will pull your chosen messages off of your mail server, which means that the only place they’ll be stored is on your Mac. You will no longer have access to them on any iOS devices or other computers you may have. Secondly, be very sure that you’ve got a backup of your machine before you get started. If you’re not careful when you’re moving messages around, it’s pretty easy to make a mistake and accidentally delete things you didn’t mean to.
Anyway, to get started with this in Mavericks Mail, it’s easiest to first organize all of the messages you’re going to archive into a mailbox of their own, assuming they’re not already. You’ll do this by choosing Mailbox> New Mailbox, and within the window that appears, pick the location for your creation and give it a descriptive name.
For the location, “On My Mac” is probably the best option. That means that the messages you move to that mailbox will be stored locally on your machine and not on your email server, which is just fine since we’ll be removing them from the program anyhow. Press “OK” on that dialog box, and you’ll see your new mailbox appear in the left-hand list.
Now you’ll drag and drop your messages to archive into that mailbox.
After you’re satisfied that everything you want to remove from Mail is in your new mailbox, right- or Control-click on it and pick “Export Mailbox” (or select it and choose Mailbox> Export Mailbox from the menu).
Choose a place to save your archive, and be certain that you check “Export all subfolders” if your mailbox has any.
The exporting process can take a long time for all you email hoarders, so to watch the progress, choose Window> Activity within Mail.
When your Mac is done exporting the mbox file, you’ll see something like the below in the location where you saved it (though you may see a different file structure depending on where you exported from and whether you included subfolders):
That’s your archive! Make sure that you see a separate mbox file for every mailbox (and subfolder) you exported, but after you’re sure that things are safe, you can go back into Mail and delete the archive mailbox you created by right-clicking it again.
OK, now you need to look at some of the messages you saved within that mbox file, so how do you bring them back? In Mail, you’ll pick File> Import Mailboxes, and then choose “Apple Mail” as the data format.
Click “Continue,” and pick your mbox file from where you saved it.
Then you’ll just press the “Choose” button, and your archived emails will all appear back in Apple Mail in an “Import” folder.
After you’ve found the information that you need, you can delete that imported folder again if you’d like to (as you still have the mbox file you saved out).
That’s pretty groovy, right? It’s like the Mac equivalent of spring cleaning, only with email…and, um, mbox files…and stuff. Hey, it’s Friday, so don’t expect my analogies to be stellar, dear readers.