Mail 3.0 Help
Archiving mailboxes
You can archive (create a backup copy) of a mailbox so you can retrieve messages if needed.
To archive a mailbox:
Select one or more mailboxes to archive.
Shift-click to select mailboxes that are next to each other in the list, or Command-click to select mailboxes that are not next to one another.
Choose Mailbox > Archive Mailbox, or choose Archive Mailbox from the Action pop-up menu (looks like a gear) at the bottom of the sidebar.
Choose a folder or create a new folder where you want to store the archives, and then click Choose.
Mail archives the mailboxes as .mbox packages. If you’ve already archived a mailbox, Mail does not overwrite the existing file but appends a number to the filename to create a new version of it.
If you need to restore messages, you can import an archived .mbox file as ‘Mail for Mac OS X.”
If you set up Time Machine to back up files, you can use it to recover previous versions of a mailbox.