What is the most portable and cross-platform format to store my email backups?

I currently use gmail, but this question is valid for any client: What is the best way to backup and store my email so that in the future I will be able to access it on a variety of platforms and clients.

Update:

While using gmail is certainly cross-platform and they have their own backup system, I am still looking to backup my email myself so that I can definitely know I have control over the backup and storage. I'm fine with letting gmail do it for now, but there's no guarantees.

The two most common open formats for storing email are mbox and Maildir.

Both essentially just put the whole email (which is a stream of [ASCII] bytes) into a file. The only real difference is that mbox puts all emails into one file (concatenated, with a separator line), while Maildir puts all emails into one directory (hence the name :-)), but with one file per email.

Both are well supported by many programs, so both will do.

Both have some advantages and disadvantages in day-to-day handling; see the Wikipedia article for details. For archiving this should not be relevant, so either will do. Mbox is not really standardized (hence possibly compatibility problems), so I'd lean towards Maildir, but both are probably fine.

my solution has been getmail using imap and backing up to maildir. i then backup the maildir with crashplan to a remote drive.

• getmail because fetchmail is buggy
• imap because gmail pop is slow
• maildir because mbox is a bad "standard"
• crashplan because remote backups are a good idea