OS X Mail Disappears - How to Fix it

I received an e-mail from my best friend yesterday and when I went to read it, it said:

"The message from "My Friend's Name" concerning “his message Subject” has not been downloaded from the server. You need to take this account online in order to download it."

But I am always online. I have DSL.

So I rebuilt the Mailbox, and then all the messages from 1.5 years of use disappeared!

Ahhhhhhh!

Nevermind what I did over the next 3 hours to figure it out... If you are having the same problem, you likely have a "Permissions" issue. How the permissions get/got changed is irrelevant. Do you want your mail back?

Here's what you do to fix it:

1. Quit Mail.app
2. Go to Home > Library > Mail > Mailboxes and Control-Click (hold down the Control Key while clicking) on the name of the mailbox that has the problems.
3. Choose Show Package Contents
4. Get Info on the file called mbox (It is likely the largest file in the Package)
5. Display Ownership & Permissions
6. Reset the Owner to Your Home account (mine was assigned to System)
7. Close the Get Info dialog
8. Re-open "Mail.app"

Your formerly blocked mailbox should be fully readable, now, with two caveats: all mail in that box is marked Unread. (No huge deal) Secondly, the mails that were giving you the message:

"The message from "My Friend's Name" concerning “his message Subject” has not been downloaded from the server. You need to take this account online in order to download it."

will still say:

"The message from "My Friend's Name" concerning “his message Subject” has not been downloaded from the server. You need to take this account online in order to download it."

So, ask that sender to re-send that message, if possible.