I'm assuming you set up an imap mail account.

When you select the Inbox or 'Get Messages', it connects to server and the imap Inbox folder synchronises with server Inbox folder to display whatever is in the server Inbox. If emails disappeared then this suggests those emails were not in the server Inbox, so cannot be displayed.

If you were to logon to the webmail account via a browser, it would confirm the contents of the server Inbox.

At this point - until it is known what is going on - Suggest you move those downloaded emails out of the imap Inbox into another imap folder for organising purposes.

Check the settings in the Imap account are as you require.

• Right click on the imap account name in Folder Pane and select 'Settings'

'Account Settings' will open in a new tab

• Select 'Server Settings'

When I delete a message....

• select 'Move it to this folder' and select: 'Trash on imap account name'

If you do not want the Trash to get auto emptied on Exit:

• Do NOT select 'Empty Deleted folder on Exit'

• select 'Synchronisation & Storage'

If you want to download full copies of emails:

• select checkbox 'keep messages in all folders for this account on this computer'
• click on 'Advanced..' button
• make sure all the folders and subfolders are selected for download.

Please note, you can choose to uncheck any folders listed, so in those folders only headers are downloaded. If you used gmail and wanted to see the 'All Mail' folder then I would suggest you do not get a full download as it is only a complete duplication.

• Then click on 'OK'

Under 'Disk Space'

• Select : Synchronise all messages locally regardless of age'
• select 'Don't delete any messages'

If you have something else selected it may explain why some messages were deleted.

Check the Inbox Properties:

• Right click on 'Inbox' and select 'Properties'
• On 'Retention Policy' tab
• Select 'Use my account settings'
• It should auto select 'Don't delete any messages'
• Click on OK

Query: Do you have any other computer or phone accessing that email address and downloading emails ? OR even another email client you previously used which is still unknowingly running in the background which may have settings to delete or are POP accounts ?

Please note: POP accounts by default will download from server and the server will delete the copies left on server. You can prevent this - in the Pop account, choose the setting to 'Leave messages on server'.

I have known someone who subsequently discovered they still had Outlook operating in the background - and the Pop account was downloading emails which were being deleted off server.

It is a common issue that many phones access mail and are often found to be using POP and deleting emails off server.

Thanks for your reply.

All settings were as you specified.

I do have Outlook installed but only for the Calendar and Tasks. I haven't got it set up for e-mail.

Curiously, this didn't happen before during the two or three weeks since I started using Thunderbird. (There was no previous e-mail client; it's a newly-built computer.)

More curiously, it didn't happen again today despite my receiving some new e-mails since the disaster.

Curiouser and curiouser...

I have taken your suggestion of creating new folder where I can stash anything that must not be lost. Maybe that's all can do, and hope the glitch doesn't happen again.

Thanks again.

What server are you using ? gmail, yahoo, comcast, BT etc

What Anti-Virus are you using ? Did it quarrantine any Thunderbird files ? If you allow AV to scan email etc, then I would advise you do NOT allow it to fix automatically because it usually causes a problem. If it does not like something, do not let it fix it. There are better and safer ways to deal with any problem.

re :this didn't happen before during the two or three weeks since I started using Thunderbird.

I'm wondering if Thunderbird started to perform a housekeeping clean up which it calls 'compacting' and you performing a download whilst this was occuring which stopped it or AV scanned a file causing the loss. Now if that occured there maybe an nstmp file in the profile - worth checking.

First create a new folder in 'Local Folders' mail account and call it 'Rescued' I'm ever hopefull :)

Do the following:

• Menu icon > Help > More Troubleshooting Information
• Under 'Application Basics' - half way - Profile Folders - click on 'Open Folder'

A new window opens showing the contents of your 'profile name' folder. * Exit Thunderbird now.

You will see 'Mail' folder - it contains 'Local Folders'

As you use an IMAP account ....

• click on 'ImapMail' folder
• click on the imap mail account name folder

Do you see any file called 'nstmp' ? If yes,

• Open the nstmp file using a program like 'Notepad++'

I'm hoping this file contains all those missing emails.

• Select all the contents.
• Copy all the selected text.

• Then go back up directory to 'profile name' folder and click on 'Mail' folder
• click on 'Local Folders' folder

You should see a file called 'Rescue' with no extension. There will also be a 'Rescue.msf' file - not this one.

• Open that 'Rescue' no extension file using Norepad++
• Paste all the contents from the nstmp file into 'Rescue'
• Save the file.

Start Thunderbird.

Thunderbird offers Calendar with Events and Tasks. You can also link up to other calendars using the built in Caldav functionality such as gmail calendar. With the additional aid of an addon I can hook up to see gmail MyTasks.

Wow, thanks for all that!

I've done as you suggested. Time will tell if it worked.

Meanwhile, since the glitch happened I've downloaded new mail into Thunderbird and nothing that was already in my In-box disappeared. So far, so good.

I'm going to mark this problem "solved." If I find out that it isn't I'll post again.

Thanks again. I really appreciate your time and effort.

BTW, to answer your question, my e-mail server is Charter.net, now part of Spectrum.

Does this mean you did find nstmp files ?

If yes, that means compacting was occuring - usually you get some message appearing in the bottom Status Bar.

Thunderbird has the ability to auto compact when it may save X amount of space. This can be switched off in which case you would need to remember to do a manual compact. It can also be set to be auto, but prompt you before it compacts. At least then you are more aware of what is going on. Personally, I choose to compact mine manually. Then I do not click on anything whilst this occurs. It does not take very long.

The trouble is this, you may have accounts that check server frequently for mail and if emails need to get downloaded or any synchronisisng with imap accounts - this will need access to a file meaning the compacting can get interrupted producing nstmp files. AND that is assuming you do not have the Anti-Virus interfering with any scanning.

The safest method is to go into 'offline' mode - do the compacting and when it's finished, go back to 'online' mode. It might sound a tad annoying, but it's reasonable quick (can be just a few seconds) or a perfect excuse to get a cup of tea/coffee.

This is where you find the settings regarding compacting:

• Menu icon > Settings > General
• scroll to 'Network & Disk Space'

Under 'Disk Space' section To set up as auto.... (uncheck if you want to do it manually - no auto compact)

• Select 'Compact all folders when it will save over XX MB in total.'

XX MB - do not set it too high, suggest not more than 50 - it may mean more frequent compacts, but is less risky and quicker.

• Select 'Ask every time before compacting'

This means you will get a prompt, but at least you know exactly when it will occur.

If you found nstmp files.... Did you manage to recover the emails which ideally are showing in the 'Rescue' folder ?

You mention Spectrum As you are using an imap mail account - you may already be aware of this but.. Note: Emails in the Trash folder are automatically deleted after 3 days. Emails in the Spam folder are automatically deleted after 14 days.

• https://www.spectrum.net/support/internet/how-delete-emails-bhn-twc

Well, I found the missing e-mails but not in Thunderbird. These are older e-mails that I wanted saved because they contain important information. A while ago, after my previous computer died and my current one wasn't built yet (still not complete, but at least functional now,) when I was connecting to the web on a borrowed tablet, I got these messages at Spectrum and couldn't print them or their attached documents. That's why I wanted to save then until I got the present rig built and a printer hooked up to it. Somehow had the presence of mind to forward them to my Yahoo Mail were I stashed them in a new folder - then forgot I'd done that. It dawned on me the other day and I retrieved them (leaving them on Yahoo Mail too, as backup.)

Meanwhile, I've had no further problems with Thunderbird. Your idea about it doing compacting at just the wrong time might be the explanation. I certainly have no other possible explanations.

I never knew that Spectrum empties the trash folder after three days. I don't think I've ever left anything in the trash folder for even three hours. I typically empty the trash folder when I'm ready to leave the webmail page.

Anyway, I can't say for certain that the problem is explained or solved, but it looks that way. Time will tell.

Thanks again for your time and effort.

Reporter's follow up question at https://support.mozilla.org/en-US/questions/1399400