Ubuntu – How to backup mail with imap, and how to read it later

I have a couple of different mail accounts that I use for different stuff,
and all can be accessed with imap (even though I use webmail a lot).

The question is if there is a way to download and backup all of those mails from the command line (a backup script), and how do you later read those downloaded mails?
Let's say one of those accounts could have been cancelled.

Does anybody have any good ideas?

Update:
Thanks jmohr for stating the obvious.

If I make Thunderbird download everything for offline use, it all ends up in ~/.mozilla-thunderbird/. And then I make sure that will be included in my normal backup when I do normal backup of my home dir.

And to read them again they are right there in Thunderbird.

So the backup will a two step rocket, first it is thunderbird updates the local dirs and then you back that up in a normal way.