In the folder with my SEA emails F:\Thunderbird_Profile\Mail\Local Folders\Personal Folders.sbd\SEA besides the folders \Substitutes.sbd and Substitutes.mozmsgs, I found the following file: Substitutes.msf When I open this in notepad++ it appears to have more recent email information though it is not coded in plain text. Sometimes there is a list of email recipients. Perhaps emails more recent than 4/18 are encoded in this file.

An msf file is an index of the associated folder; the actual messages are in a file with the same name as the msf file, but no file extension - the mbox file. If you delete the msf files, they will be automatically recreated when you restart TB, and this sometimes fixes the problem of a missing folder. It is the same as right-clicking a folder in TB, Properties, Repair Folder.

Thanks for your quick reply.

In Windows Explorer in this directory:

F:\Thunderbird_Profile\Mail\Local Folders\Personal Folders.sbd

I verified there are two files: SEA and SEA.msf

I opened up SEA in notepad++ and verified there were emails from at least as recently as November. But when I opened Thunderbird, right clicked on the SEA folder, clicked "properties" and then "repair folder" the missing emails did not appear.

In addition, in the subdirectory SEA.sbd, I found a file Substitutes.msf but no file Substitutes.

The mbox and msf file pair are always in the same location, so if there is no Substitutes mbox file, the messages contained in it are apparently gone. I think the fastest and most reliable way to fix this is to create a new profile with Profile Manager, set up the accounts, then copy the mbox files into the Mail/Local Folders subfolder of the new profile. If the old profile has a complex folder structure, you can instead try using the ImportExportTools add-on, but if a folder (mbox file) is corrupted somehow, it probably won't appear properly even in the new profile.

I found a recent Substitutes "mbox" file in my Microsoft File Backup external hard drive. I also checked the SEA "mbox" file and it had the same size as the backed up version so it is probably OK. I copied the most recent Substitutes "mbox" file to the correct location in the Profile. I deleted SEA.msf and Substitutes.msf. Then I ran Thunderbird. This seemed to recover all the emails except perhaps for the last week's worth. Question: It seems that emails are also stored in .mozmsgs folders as separate .WDSEML files. Did TB go over to a new way of storing emails? My most recent .WDSEML files mostly seem to be from April, 2018 though there is one from 12/30/18 though strangely enough the email is from 2014.

The wdseml files are created for Windows search:

http://kb.mozillazine.org/Importing_folders#Find_the_mail_folders

Except for maildir-type storage in a non-standard TB setup, complete messages are only stored in mbox files.

Thanks for your help. I can live with what I have.

Since I rely extensively on my email archives for my work, it's crucial to me that my archive is reliable. I feel fortunate that I had a recent backup and I could backtrace and recover most of what was lost.

Is there a method that I can use to periodically check to verify that the archive, including folder structure and the emails themselves, have not been corrupted?

Thanks again.

I don't know of a single method to verify an archive, but most of the advice here is relevant to avoiding integrity problems in the first place. Besides regular backups of the Thunderbird folder (the folder that contains the Profiles folder, which contains the individual profile folders), you could also use an archiving program like MailStore Home as a second storage program. For IMAP accounts, I've found it useful to occasionally rebuild the Inbox by backing it up and then deleting the mbox and msf files, which are automatically recreated and repopulated upon a TB restart. This works well when a folder is still readable, but 'asks' for frequent compacting, or the folder size reported in TB is very different from the size of the mbox file.