Backing up your email
You can back up your mailboxes to a CD or another disk. To do this, you can either copy the Mail folder in your Library folder or create an mbox file, a common format for storing mail messages.
To save the contents of all your mailboxes, hold down the Option key and drag the Mail folder (located in the Library folder of your home folder) to your desktop or to another disk. Copying the Mail folder only backs up mail from POP accounts, or mail from an IMAP account that you've copied to local mailboxes. When you are ready to restore the Mail folder, choose File > Import Mailboxes, select Mail for Mac OS X, and locate the Mail folder.
To create an mbox copy of a mailbox, drag the mailbox from the mailbox list to your desktop.
To create an mbox file with just some of your messages, select the messages, then choose File > Save As. In the Format pop-up menu, choose Raw Message Source, then enter a name and save the file.
You can copy mbox files to CDs or to other disks. When you want to restore your mail, choose File > Import Mailboxes, select Other, click Continue, and locate the files.