Update: The individual .msf and mbox files that contain each subdirectory's emails can be viewed with Notepad. Opening a few shows that the emails are still present.

As a quick test, I created a second Archive2 directory under Local Folders and copied across to it a single sub directory from the original.

The subdirectory shows up, but again, Thunderbird says that it contains no messages, even though there is clearly data within its .msf and mbox files.

I then tried renaming the .msf files, leaving in place just the mbox files (in both the original Archive and the test Archive2 directories). The new .msf files that are created are zero bytes in size (as against 83KB for the originals) and contain no references to emails contained in the mbox files.

The SDB file creates the structure for subfolders ie archive.SDB creates a location for Thunderbird to store sub folders of the archive folder. It in no way is involved in the creation or display of the archive folder.

The msf file is entirely transitory and while the metaphors are largely mixed it represents an index of mail in the Thunderbird folder it is named for and contains sufficient information to load data into the user interface. It is not the mail. You can delete them and they will be automatically regenerated, although some things like the sort order are lost with that process.. Thunderbird will recreate the folder lists and mail lists without them.

You mail and the critical part of the import is the MBOX files and these files have no file extension at all. You mention them, but have you looked in them with a text editor? (I recommend notepad++ simply because it come with the huge files MBOX represents usually much better than the Windows notepad)

The layout of an MBOX file is largely a stacked one after the other representation of the text that forms the email as it travels the internet. Open the file and examine the X-Mozilla-Status: for a few of the emails. What is set in there? Normal read emails have a X-Mozilla-Status: 0011

The fact you are generating Zero length index files indicates there is nothing in the MBOX file to index, so I would be guessing the status is actually set to deleted. But have a look!

The other newly appearing issue is folks leave the quick search toolbar open (Ctrl+ Shift+K) with the unread or search criteria in the search and they see no mail as this information has not been cleared and the folder content does not match the criteria.

Thank you for your response Matt.

As mentioned in the 2nd Update note, I have opened the mbox files with Notepad, and can see that they contain messages. It does appear as if all of the archive is actually present, just not being displayed.

Here are some examples of the X-Mozilla-Status of messages in one of the mbox files.

X-Mozilla-Status: 0009 X-Mozilla-Status: 000b X-Mozilla-Status: 001b X-Mozilla-Status: 101b X-Mozilla-Status: 0019

After a search around I found reference to some of these codes: https://forum.geekzone.fr/t/perte-de-tous-mes-mails-sous-thunderbird/20084/5

According to the info in this link, the last 9 or b indicates that the message is set to 'undisplayed'. I changed the status of one message to 0001 and it is now visible.

The majority of Status2 are set to 00000000 but there are a couple set to 00800000, or 10000000. What does Status2 signify and might these need to be edited ?

Can you think of any reason why, suddenly, hundreds of messages in an archive might have had the status set to 'undisplayed' ? It's very strange.

And also, do you know if there is any way of resetting resetting hundreds of status' to 'visible' without having to manually edit many dozens of mbox files ?

Thank you for your response Matt.

As mentioned in the 2nd Update note, I have opened the mbox files with Notepad, and can see that they contain messages. It does appear as if all of the archive is actually present, just not being displayed.

Here are some examples of the X-Mozilla-Status of messages in one of the mbox files.

X-Mozilla-Status: 0009 X-Mozilla-Status: 000b X-Mozilla-Status: 001b X-Mozilla-Status: 101b X-Mozilla-Status: 0019

After a search around I found reference to some of these codes: https://forum.geekzone.fr/t/perte-de-tous-mes-mails-sous-thunderbird/20084/5

According to the info in this link, the last 9 or b indicates that the message is set to 'undisplayed'. I changed the status of one message to 0001 and it is now visible.

The majority of Status2 are set to 00000000 but there are a couple set to 00800000, or 10000000. What does Status2 signify and might these need to be edited ?

Can you think of any reason why, suddenly, hundreds of messages in an archive might have had the status set to 'undisplayed' ? It's very strange.

And also, do you know if there is any way of resetting resetting hundreds of status' to 'visible' without having to manually edit many dozens of mbox files ?

Historically one of the "best" summaries of the x-mozilla-status has been this page from one of the early Mozilla developers http://web.archive.org/web/20070126161519/http://www.eyrich-net.org/mozilla/X-Mozilla-Status.html?en

But you could always go to the source. https://searchfox.org/comm-central/rev/b904d8acfdd5572ff6b9e8b519b0db7c58271fb6/mailnews/base/public/nsMsgMessageFlags.idl

The flags are defined in the file nsMsgMessageFlags.idl as hexadecimal values.

So your message with a 9 would be a deleted read message 1 + 8 basically

It is close to thirty years since I did any of this regularly, so memory of some things is not as good as it was and hexadecimal conversions to binary to set flags is at least that old and I never did get those high order and low order bytes clear to myself, so I might have this binary backwards. However here goes my attempt to explain the 000b status

X-Mozilla-Status: 000b is binary 1011 Deleted 0x00000008 = Bin 1000 So that accounts for the first 1 Read 0x00000001 = Bin 0001 So that accounts for the last 1 Send Successfully 0x00000002 = Bin 0010 And that is our second last binary digit

So the message flagged as 000b was read, sent and deleted. The assumption about last digits and display is I think just that, an assumption that really has no basis in fact. Setting the status to 1 will make them appear as read as the source shows.

I have no idea why this could suddenly occur specifically, but if the folders are stored on an IMAP server in an IMAP account (and as such are not really local archives) simply loosing connectivity to the server can see everything in the local store set to deleted pending a new refresh from the canonical store (The IMAP mail server) as to what the folder contains just as would occur with an IMAP inbox or other folder.

Just a note of explanation. Your posts have been flagged by the system as spam and hidden because you are a "new" user and used links in your post. I have cleared the spam flag on your messages and they should appear now. Hopefully you should be right now regarding spam flagging.