Mail quits on attempt to access one particular Inbox

Question

I have a Mail problem localized to just one of my account Inboxes.
Using the arrow to the left of Inbox, I can view my separate Inboxes (I have three POP accounts). My two other account Inboxes, and all my other mailboxes, preferences, filters, etc., work and display normally.
I can successfully POP the mail for this problem account (saving copies on my server for safety), and the "new messages" number to the right of its Inbox increases -- but as soon as I click on its Inbox to view it, Mail quits.
I have deleted all my .mboxcache.plist files (using FileBuddy), but on restart, Mail appears to create new copies of them and the same problem happens.
Based on related discussions in this forum: I have no third-party extensions to Mail, I've never installed any new fonts, and this problem started after my system froze because it had run out of disk space but an application was still trying to write to the disk (so I expect the cache or index of this one mailbox didn't get saved properly when Mail was forced to quit).
This Inbox is too important to trash and unfortunately my most recent backup of it is almost a week old, so missing important recent messages. And I do not think it's a problem with the messages, but with the caching or indexing of them.
Problem can't be solved by importing (when I try to import my mailboxes into a fresh copy of Mail, it does not import any Inbox content from any of my three accounts).
Is there a way I can fix this Inbox? Thanks very much for any thoughts!

Answer 1

You're welcome.
Copy the old INBOX.mbox moved to the Desktop and place the copy in another location of your choosing such as your Documents folder for backup purposes.
Repeat Show Package Contents for the INBOX.mbox on the Desktop and delete all package content files except for the mbox file at 33.9MB in size and empty the Trash.
This is MB and not GB correct?
If so and with the Mail.app quit, using the Finder go to Home > Library > Mail > Mailboxes.
Move the INBOX.mbox from the Desktop within the Mailboxes folder.
Launch Mail and this mailbox will be available under On My Mac in the mailboxes drawer.
Select the mailbox in the mailboxes drawer and the Mail.app will take additional time to re-index the mailbox. If successful and when completed, all messages in the message list for this mailbox will appear as unread.
To mark all messages as read, select a message in the message list and at the menu bar, go to Edit > Select All. At the menu bar, go to Message > Mark and select As Read.
Use the Rebuild Mailbox function on this mailbox for good measure.
If there are no missing messages and this resolves the problem, you can delete the backup copy of the INBOX.mbox.