Forgot to mention that email service is via Verizon. Went back to Verizon webmail to see if I could rescue anything but the account is set to delete emails from the serve 2 weeks after being downloaded by Thunderbird.

http://kb.mozillazine.org/Disappearing_mail

Thanks - have read through that pretty carefully and none of the options that I've been able to figure out have worked. Then again I'm not skilled enough technically to run all of them, so it's possible one could work and I just haven't been able to figure it out.

Before trying anything else, do you have a recent backup of your profile folder?

Unfortunately not. It's the office mgr's machine & profile, not mine... not that I've been particularly good abt backing up my own. Lesson learned, of course.

check the anti virus quarantine. They have a habit of quarantining and if your settings are right/wrong deleting detected malware. Unfortunately when it is in the say, inbox, they delete that as well, or at least quarantine it.

Thanks for the suggestion. Will check the sandbox and the settings tomorrow morning. We use Avast Free. Haven't changed that in years, but maybe something got changed unwittingly...

Nothing in the virus Chest that indicated Avast might be the culprit here...

Nothing in the logs either?

What size in the MBOX file in question? I just re read this, have you tried the simple right click of the folder, aelect properties and then repair? We have been off on the big stuff and ignored the obvious.

I did try the repair .msf option first thing, and tried it again today. No results so far.

I don't know how big the MBOX file is - I wasn't even sure I was in the right one when I was looking at them.

Weird thing - we moved just a few pieces of more recent mail (mostly unimportant things from late 2013) into the folder last Thursday, and they are GONE today as well. Most recent email shown currently is 11/6/2012. Very strange.

What folders are we talking about, and from where to where are you moving messages?

What does 'one fold had been renamed clearly in error' in your OP mean?

Sorry, I'm not great with the terminology and had a few typos to boot.

Typically every week to two weeks, emails in the Inbox are moved to a Correspondence Files folder, in which we have a separate subfolder for each client. Last week our office manager (who uses this computer and email) noticed that roughly 16 months' worth of email was missing from a client subfolder, from the most recent emails back to 11/6/2012. This is a client that generates a lot of email activity, so I would guess that somewhere in the range of 300 emails have gone missing from the client subfolder. In addition, the affected client subfolder had been renamed, ie instead of being named "Client Name Here, Inc" it was now named "Client Na29c46T8" (number/letter sequence at end not exact.)

I do not know if the two issues are related, are caused by unwitting human error, are caused by a corrupted file, are caused by poor maintenance on our part, or are caused by some combination of the above.

With regard to poor maintenance: The computer operator tells me that she's never been prompted to compact files in Thunderbird. She had also never emptied her trash file, which was up to 4,500 emails. From what I understand, compacting is automatic.

We migrated to Thunderbird almost exactly one year ago (from Outlook).

Computer is running Thunderbird 24.4.0 on Windows XP.

Anything else I can add to help clarify?

I think at this point in time corruption is almost certain in that particular folder and the only real recourse is deleting it.

On the same pane as the "Repair folder" is the size on disk, and an absolute location on disk for the folder. You need to trim the mailbox:/// but if you copy the location using your mouse and drag you can copy that location with Ctrl+C once it is highlighted.

Go to Explorer and paste that location into the location bar and you will be sure your in the right spot. Close Thunderbird Copy the file named the same as the folder in Thunderbird. Not the one with the MSF extension the larger one with no extension that is about the "size on disk". And past it somewhere safe. (That is a backup of the folder contents.)

Start Thunderbird Drag all the mail out of the folder to a new one. Delete the existing folder (It has the wrong name now anyway)

I would like to know the size on disk before you do that however. If it is 4Gb or close another plan should be used.

clearly renamed in error. I have no idea who or what. But you say named "Client Name Here, Inc" it was now named "Client Na29c46T8" that is clearly in error in my book.

"What folders are we talking about, and from where to where are you moving messages?"

Your moving all the messages out of the corrupt folder that keeps loosing them. I don't care where, just move them somewhere. Make a folder and put them in it.

To:  well I assume once you have deleted the existing folder you will recreate the folder and move them back so it is rather redundant,  where you can find them would be my suggestion.

Is that what you wanted to know?

Folder size is currently 481 MB.

Matt, currently moving all mail from old client folder to new client folder. When done, I will delete old client folder.

Do I need to delete the old client mail file as well, or will that be deleted automatically when I delete the folder?

Should I compact as well?

Have we at this point given up hope on recovering the missing mail that started this issue altogether? If so I will go ahead and migrate thunderbird mail account to new computer.

Thanks for all the help, much appreciated even if we don't wind up recovering the missing mails...

Matt, currently moving all mail from old client folder to new client folder. When done, I will delete old client folder.

Click the first mail, hold shift click the last mail and drag all at once. Ctrl + A also selects everything in a list.

Do I need to delete the old client mail file as well, or will that be deleted automatically when I delete the folder?

cut and paste it out of the Thunderbird location to say your desktop. Location is not important, but it does leave the oportunity to poke around in it.

Should I compact as well?

Have we at this point given up hope on recovering the missing mail that started this issue altogether?

Now back to the folder you copied out. Remember it is the file with no file extension, the large one that sits beside the MSF file with the same name.

This is a text file, beginning with the oldest email and ending with the newest in text folmat. so open it up in a text editor and have a look and see what is physically there. If there is nothing, there is nothing. If you can find something that looks like email there is hope.

A second line of possible recovery is wdseml files. Thunderbird creates them if the question to allow windows search to search the mail was answered yes the first time Thunderbird was run. But again search the profile for them is they exist recovery is possible. If they do not, another dead end.