You could rather quickly and easily write a shell script to:
- use offlineimap [0] or imapsync [1] to perform a local sync of your mailbox to local mbox files or maildir directories,
- (optionally) use tar or similar to create a single file out of your entire mailbox, and
- use tarsnap [1] to back it up (to AWS) or s3cmd [2] or similar to back it up to wherever you want it.
I used to do exactly that with an additional step to encrypt the tarball (with GPG) before uploading it. It worked out quite well.
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[0]: http://www.offlineimap.org
If you're using s3cmd to back up your mail then the "create a tarball" and "encrypt with GPG" steps are useful, of course.
The majority of serious email get copied (IMAP or for the email server we run, just sftp) to a pair of DVDs on periodic basis as an encrypted filesystem archive. We then stick it in a dvd swapper drive for offline access. The other copy gets put off site.
For home use just get a bunch of usb dvd drives and go nuts, or go full tape. Once you go tape you get addicted to capacity. But it gets pricey. Cost/density is interesting.
At home I just use a stripped down version. Literally a bunch of usb dvd drives hooked up to a raspi. I can insert a disk and poke around or have it auto copy the whole thing and auto-eject. I have 10 slim drives connected... ebay is kind. The raspi pipes to a different server full of ssd/hd/ram depending on what I'm doing.
Just a computer with a LOT of dvd drives. Add a few high capacity local ssd, harddisk and plenty of ram. Insert disk, optional full copy to local as cache, then server just reads. If it copied it auto-ejects.
Also got a tapes as a backup. Works similarly. Insert tape, it gets copied to the libraries local storage then ejected. We can also just use the tape without copying first.
Why optical for email? We can easily store them in a human accessible way. They're supermarket cheap. Quicker than tape as well. Everything on optical is also on tape with no exclusions.
Its a pity optical media hasn't reached the point where I can just go buy a pack of 1TB disks from the supermarket. I'm not holding my breath for Sony's Archival disks either.
they have an awesome free trial running right now..