re : The repair instead deleted all of the emails except Jan 8, 2018 - April 18, 2018 - about 890 emails left.

No, repairing an index file does not delete anything. The repair only repairs the index. In other words , it looks at the file which has the emails and rebuild the index to match what emails exist. It would seem the emails had already been lost quite some time ago.

re :The file was fine as far as I know on 4/19 when I backed it up with the 6000 entries in it, but that same inbox, restored to another computer, also only shows the 887 2018 emails.

Clearly, it did not have 6000 emails. The index was already not displaying the correct figure. Something had already occurred. So, there are only 887 emails. You know this because you repaired the folder and it also said there was 887 emails.

What is not known is how many hidden 'marked as deleted' emails are in the original backed up file.

re : I tried rebuilding the msf file manually by deleting it and reopening Thunderbird but that did not help.

The .msf file is the index file. You repaired the index by forcing TB to create a new .msf file and rebuild based on what exists in mbox file.

re :I tried reinstalling Thunderbird. Not sure why you would that as the Thunderbird program is not at fault and it would still be using the same profile folders. There are very few instances when the Thunderbird program is at fault.

re : Opening the file in notepad the earliest date I see is Jan 24, 2018. I presume that data is at the top of the file.

re :tried to repair the inbox again. When I did that, all of the Jan-April emails disappeared too. As the third repairing of index file appeared to remove more emails, something else is going on. It is possible something has scanned that file on start up before you had a chance to do repair.

The Inbox folder is designed to be the inbox for incoming new mail, it is should not be used as a huge storage file for old mail. All older mail should be sorted in to suitably named folders for storage and organising. The Inbox gets a load of activity, new incoming mail, deletions, moves to other folders etc. All emails arrive and are written to the file one after the other, like each email is a paragraph in a long document. The oldest will be at the top. When you delete an email, it is 'marked as deleted' and hidden, but it is still in the file. When you compact the folder, it removes all sections containing 'marked as deleted' emails to reduce the file size. If some corruption occurs, then it is possible to lose track of all the 'marked as deleted' sections and it is possible to lose emails.

You have not mentioned whether you compact the Inbox and Junk folders on a regular basis (I do it once a day, but it does depend upon how many emails you delete or empty from junk folder). Perhaps, you have the compacting done automatically. Maybe a threashold setting is used to tell Thunderbird to compact when it will save XX space. A lack of compacting on a regular basis can allow files to get corrupted.

* Exit Thunderbird if it is running - it must be closed.

• Access Anti-Virus product and stop it scanning Thunderbird files and folders on startup.
• Stop AV scanning incoming and outgoing mail.

• Do not allow 'CCleaner' or any similar product to have access to any Thunderbird file or folder. They cause havoc.

• Suggest you open the original 6000? email file from backup using Notepad.
• Look for this line: X-Mozilla-Status:
• Edit as required - Make sure it has this number 0001 - see images below.

If some have number 0009 - these could have been marked as deleted.

• Save the file.

• Access the correct mail account
• Rename current Inbox as 'OldInbox'
• Rename the 'Inbox.msf' file to say 'OldInbox.msf'

• Copy and paste the backup 'Inbox' mbox file into correct mail account.

• Start Thunderbird.

• Create suitable folders.
• Move all good emails out of 'Inbox' into those other suitably named folders.
• Delete any emails you do not want.

When 'Inbox' is empty :

• Right click on 'Inbox' folder and select 'Compact'

• Right click on 'Junk' folder and select 'Empty Junk'
• Right click on 'Junk' folder and select 'Compact'

This will start with a clean fresh Inbox. All emails you need to keep should be in suitable folders. If the 6000 backup did not have a load of 'marked as deleted' emails, then it is likely they were gone a while ago, but not noticed.

This is an excellent explanation and I really appreciate your taking the time to answer my question. I will follow your additional instructions. Especially the anti-virus. I will make sure those files and folders are excluded, and this PC is not running CC Cleaner. One thing that still bothers me, though, is that when I initially opened the inbox - the one with the 6000 emails, where some of the body text was missing - I had many full emails from 2016 and 2017, and even successfully forwarded a bunch of the prior year emails to myself. It was after I did the forwarding that I did the repair to try to get back the older ones - and that is what resulted in an inbox with nothing but 887 2018 emails. Nothing from prior years at all. Because of the file size, I just have a feeling those are still around. From your answer, I am not quite sure if there is a chance that they are. These are mostly text emails with our clients - we are accountants - not lots of graphics or attachments that would make for a huge inbox size. This is actually my husband's computer, he knows nothing about properly managing his files, so I was attempting to do that for him by moving out a lot of those old messages into other folders. I am quite sure he never marked any of those emails for deletion. And I do know for a fact that emails prior to 1/8/18 existed in the inbox before the repair because I forwarded some to me. Again, thank you very much for this help.

Is the mail account a POP or IMAP mail account?

It is a POP account.