I don't quite believe this... For a long time I have taken backups of important mail in Mail.app by first archiving them in a folder in the Mail.app left panel and then just dragging them to the Desktop. This way I've had an mbox file which I could burn to CD or import back to Mail at will. Or so I thought...
When I today tried to import some of my archived mail from an mbox file I noticed that I simply couldn't do it. I could open individual emails from within the mbox file, but could not import all of them back to the Mail.app. At this point I turned to the Internet and found this explanation on a small developer site:
'In Mac OS X 10.4 "Tiger", the default message format for Mail messages changed from the Apple custom mbox-package format to the new emlx format (where messages are stored in individual files for Spotlight indexing). However, if you need to recover from a hard drive crash, it's almost impossible to recover your mail messages easily since Mail won't import emlx files and you can't add them to your mailboxes any other way.'So, in order to get Spotlight working, Apple decided to lose easy mail archiving. Note, that there is no way of exporting or saving mail from the Mail.app – at least I don't know about it.
Luckily the same site that offered the explanation also provided a solution in the form of a small program which converts the new format back into standard mbox format which then can be imported back into Mail. Brilliant. If you need the app, you can get it
here.
But really, Apple should've done this, or at least they should have provided an other way to export and import your mails. Not good, Apple.