Mail Local Folders Empty

Yesterday I tried to move my locally stored emails from my old macbook to my new macbook. I went into Library > Mail > V2 and copied all mbox's to a hard drive and then replaced this folder on my new computer.

I can now see all the folders but there are no emails in the folders.

I've obviously done something wrong, has anyone experienced this before and know of a way around it?

When I try to import via the import mailboxes option in Mail, when I select the right mbox, it says no valid mailboxes found, which is strange

 

Found the answer

1. Quit Mail
2. Navigate to ~/Library/Mail/V2/MailData
3. Delete any file that starts with "Envelope Index"
4. Start Mail
5. Follow prompts to allow Mail to rebuild the index -- this may take a few restarts of the app.