It sounds like you have an IMAP mail account. Imap mail accounts will only show what is on the server. Hence why you are experiencing emails deleting.

Imap mail accounts see a remote of the server folders. Headers only are downloaded and when you select to view an email it is retriebed from the server and stored in a temporary cache to faciltate viewing. It is not stored in Thunderbirdprofile folder.

synchronization of subscribed folders for offline use downloads a copy of the email and stiores it in Thunderbird profile, but these folders still synchronise to show what is on the server.

If you keep the imap mail account, you need to make sure your subscribed folders are synchronised for offline working so that a copy is downloaded. Info here:

• https://support.mozilla.org/en-US/kb/imap-synchronization#w_configuring-synchronization-and-disk-space-usage

then go into offline mode to temporarilly stop synchronising. right click on highlighted emails and use 'Copy-to' and select a suitable folder in 'Local Folders' mail account. Check you can read some of those copied emails to verify they copied ok.

Go back into online mode. Emails stored in 'Local folders' are stored in the Thunderbird profile folders and do not synch with imap folders. So, you should remember to perform backups of profile.

The alternative would be to create a POP mail account. It depends upon whether all emails come into Inbox and you sort them or whether someone else has the ability to sort them as well. If it is only you who have access to those emails, then you could use the copy to method to get all the various imap folder emails into Local Folders and from then onwards use POP to download directly from Inbox all new emails.

Pop mail accounts can only get access to the server Inbox folder. Emails are downloaded and stored on your ccomputer in your Thunderbird profile. Before downloading any emails, you would need to make sure the Account Settings > Server Settings are set to 'Leave messages on server', so that you do not delete anything.

Yes it is an IMAP server. I've called the WSU mail support folks and they have no idea how the save mail area of The IMAP server and therefore the Thunderbird area got deleted. I did find they have an ALL MAIL section that has all of the deleted mails are located within. The header i on each shows the folder it came from so I can go back through and 1 by 1 put them back - all 2000 plus mail items. This will take a week or more to finish. Because "nobody" seems to know how this happened, is there a way to make matching folders on my machine and save the mail there so that WSU or Thunderbird program can't start a wholesale delete operation again? I want these mail entries protected from everybody but me - I DON'T CARE about about how much disk is used space used. BIG disks are cheap and the mail is unreplacable. Thanks

If wsu have a policy of wiping messages to free up disk space, then you need to rescue messages and store them yourself. I'd be copying or moving them to Local Folders in Thunderbird. This detaches them from the server and makes them immune to any server maintenance activities.

The mention of "All Mail" seems to hint that they are using Gmail. Periodic wiping would then be unusual, since Google are somewhat retentive and discourage deletion. Their "All Mail" folder which retains deleted messages is testament to their reluctance to allow you to delete anything.

They are using Gmail. I didn't know about the local folders and that seems like a good solution to my problem. I think the real problen may be that my file system that is broken and is causing the rest of the problems. I had gone through over 1 thousand mail entries in the ALL MAIL folder and those were all back in the proper folders. When I created my first new folder under local folders everything I had already saved - all went away again. Thank goodness that they were all in the ALL MAIL folder. After I have recreated the folders under the local folders. Is there a way to save all of this so if after I resolve the file system problem, I have to reload the Thunderbird application, I can reconnect the local files. This way I would not loose that information again?

I seems the most important thing would be to get a copy on your computer first. As everything is in the 'All Mail' folder, I would subscribe to see 'All Mail' folder and make sure it is synchronised for offline use so that a copy is actually stored in Thunderbird profile folder. Then go into offline mode and use right click on highlighted emails and select 'Copy-to' to get a copy in your 'Local Folders' account folders. Whilst in offline mode check you can read those emails in the various folders in the 'Local Folders' account. If you can read those emails then you know they are copied over correctly. This will at least get a copy onto your computer, which you can backup to an external device.

It is unlikely that uninstalling and reinstalling the Thunderbird program will have any effect.

As this is an imap mail account, I would remove the imap account and then create it again. This will not have any effect on emails stored in your 'Local Folders' mail account.

Additional: Before recreating the imap mail account, make sure all the emails are in the correct folders on the server/webmail account.

Thanks for the information. The first thing I did was - IN the WSU email account I reduces the 2600+ mails down to 563 that I needed to keep. Next under the LOCAL I created a duplicate set of folders and subfolders that matched the ones in the WSU mail system. Then I copied I copied individual mails from the ALL MAIL to first the non local folders and then to the duplicate local folders. I have checked all and because I did this over several days I sometimes forgot where I left off and had some double entries in the local folders which I had to delete 1 of the copies. I use a stand alone program that makes a total stand alone copy of each of my machines on the first of every month to a BIG USB attached HD. They contain all of the backup copies for the past year, I actually have 2 USB HD’s that are duplicates so if one goes bad, the other has the same data. I think my problem is now under control and this can be closed. I am now going to install Thunderbird on another of my machines and would like to still move a copy of the local folders I just created to the new Thunderbird on a different machine. I have decided that my browser – SeaMonkey, another Mozilla product, is part of my problem and am converting to FoxFire. I am going to try and see if the information in SeaMonkey can some how to moved to FoxFire or another week will be required to get FoxFire operational. I will do more reading and may have to open a question there also. I expect that will be a different Mozilla support group

My problem is resolved. I also installed Thunderbird on my Windows 10 machine and ended using Mozbackup to export the settings, filesystem and mail from the machine I was having problems with to the new installation on the Windows 10 machine. all is working GREAT... Thanks you for all of the assistance - you helped me resolve the problem