How to recover missing content of mail folders after upgrade to Maverick?

Question

When I upgraded my Mac computer to Maverick I lost the content of my mail folders.  The folders are there but there are no emails filed in them.

Answer 1

Try to re-index:
Repair & Reindex All Messages in Mail app
You may encounter an alert from Mail app telling you that the mailbox must be repaired, but you can also do this manually. This is particularly helpful if the search functions of the Mail app have become unreliable.
Quit out of Mail app and go to the Finder
Hit Command+Shift+G and go to the following path:
~/Library/Mail/V2/MailData/Delete every file that begins with “Envelope Index” (optional but recommended: back up these files to the desktop just in case something goes awry)
Close the MailData window, then relaunch Mail app to force reindexing
Like rebuilding the mailbox, the reindexing process can also take a while, depending on how large the mailbox is and how much mail is stored on the computer. Be prepared for a lengthy re-indexation if you have tons (thousands+) of messages in Mail app. When finished, try the search or perform the task that you had issues with before and things should be working as usual again.-These two solutions will fix most common mailbox related issues encountered in OS X Mail, so try them out the next time Mail app is performing strangely. On the mobile side of things, iOS does not have similar forced rebuilding and reindexing options, and thus to resolve similar issues you often have to remove the account from iOS Mail settings and then re-add it to resolve like problems, but that’s a topic for another article.