Backup IMAP messages?

Question

What is a good strategy for backing up mail messages (and mailboxes) stored on an IMAP server, such as .mac? We have quite a few mailboxes, and thousands of messages for our small business.
We have a few computers with synchronized access to the same .mac collection of mailboxes. A concern is that, in the event a mailbox or message on .mac's IMAP server is accidentally deleted, the deletion would be copied to all the other machines.
So we would like to keep a backup.

Answer 1

Thanks for the tip.
My backup scheme for IMAP boxes (in panther OS X 10.3) is this:
I drag an IMAP folder to the "on my mac" section, which appears to copy the messages in the folder, so they now exist in the IMAP folder hierarchy and on the 'on my Mac' hierarchy. I then drag the folder to the desktop, where it is saved in mbox format. I then delete the new folder I made, as I don't want both copies cluttering up the database.
When I want to look for something, I import the saved mbox, or a folder full of mboxes, and they are automatically put in a folder called 'Import'. When I have found what I want, I copy it to another folder in the mail program, and delete the rest of the imported mailboxes.
(However, Andreas Amann says you can't import mbox files in Tiger so this scheme may already be out of date if you don't want to use his work around.)
another alternative, described by Fernando Schlot (Jan 15):
Select all the messages in a mailbox. Choose 'save as' and choose the format you want. This saves a single pdf file with all the messages one after the other. This file can be searched as a normal file in the finder.
The pdf format is smaller (3 mb for 140 messages, against 4 mb for the mbox format) but they can't be reimported.
A third alternative (Schlot again) is to make a new user (call it something like Mailarchive2005, so you remember) and copy the ~Library/mail and ~Library/preferences folder to this new user. Now all your current mails should be in this new user space as well as your normal one. Go to the Mail Preferences/General, and switch 'checking new mail' to 'manually'. (This is to avoid current mail being downloaded automatically into your archive as you search for an old message.)
Now you can clean out all the mails from your normal mail programme in your main user space and start afresh. When you want to find a mail from last year, just switch to the Mailarchive2005 user space, look for the email and redirect it to yourself. Switch back to your main user space and download the mail. Alternatively you can save it as a pdf to your hard disk, top level, where it will be accessible to both user spaces.
I used this system with Outlook Express, where you can make new identities within the programme. Each year I made a new identity.
powerbook G4   Mac OS X (10.3.8)