[old-gregg]

mbox is a standard email storage format. It's not ideal for a live email system, but not bad for archiving or for import/export purposes.

You can import mbox into variety of desktop email clients and, using IMAP, upload them to web-based UIs.

Here's how to move off Gmail to Rackspace (not free) email, I've done it last weekend [1]:

  - Enable IMAP access in Gmail
  - Add Gmail account to Apple's Mail.app
  - Wait for Mail app to sync.
  - You have a full copy of GMail archive on your computer now
  - Select all messages in Mail app, and select Message/Copy To
  - Pick Rackspace inbox and click "OK"
  - Delete all your email messages in Gmail, purge Trash. 
  - Done.

I also exported Gmail labels into separate mbox files just in case.

[1] http://www.rackspace.com/email-hosting/webmail

P.S. For anyone wondering, I moved off Gmail because of two reasons:

  1. New compose window was impossible for me to use.
  2. Security issues.

[Groxx]

I've done similar things a variety of times, the download -> copy -> paste routine is pretty simple. So far I've never had a problem with it (even moving 100k or so).