We do backup our mail, although it's not necessarily easy to download in a tar archive.
We also provide IMAP access to mail, which means it should be pretty much drag and drop to copy a folder or folders to your local machine if you have a GUI mail client running.
In the future, you would be (easily) able to create a tar archive of all your mail from the command line, but note that it will be in Maildir format instead of the standard 'mbox' format - with Maildir, each message is in a different file instead of in one big file.
This is faster and easier for our servers to handle, and is less likely to become corrupt, but is a bit less portable.
I hope that answers some of your questions (at least indirectly).
We keep "snapshots" of mail for at least a week or two, generally longer, and you can easily make your own backups in one way or another.