Problem migrating mailboxes to Mountain Lion I have just set up a new mac and migrated the user accounts from the old system (Snow Leopard) to the new one (Mountain Lion) and everything went well except for some parts of the Mail setup.

In the old computer there were several mailbox (.mbox) directories on the Mac itself.  These mailboxes were transferred to the new computer and it looks like they were successfully processed by the migration into a new directory structure.

However the mailboxes do not show up in the new Mail program.

(Note that the mbox's directly related to the email POP accounts did successfully transfer to the new mac)

For example, there is a mailbox called Travel and for a particular email message it is located in the old and new systems as:

/Users/oldmac/Library/Mail/Mailboxes/Travel.mbox/Messages/24307.emlx
/Users/newmac/Library/Mail/V2/Mailboxes/Travel.mbox/A24F8F9F-8AEC-4802-9FF9-10A497A5352D/Data/4/2/Messages/24307.emlx


And the text in 24307.emlx in the new location matches that of the old location.

So the question is how do I get these local mbox folders to appear in the Mail app?  Do I have to manually configure something under the hood, or is there a user configuration somewhere that I have missed?
Have you tried doing like this?


Navigate to: Macintosh HD → Users → Your User Folder → Library.
Now you need to copy the Mail folder, as shown below. (Yes, the entire folder.)
You should also copy the preferences file for Mail.app. Here's where you can find it: Macintosh HD → Users → Your User Folder → Library → Preferences. The name of the file is com.apple.mail.plist.
Copy the Mail folder and the preferences file to the Library and Preferences folder. Make sure that the folder and file are in the correct location!


Source: 
http://www.macinstruct.com/node/183