Update: ok, my little test msg "sents" are gone again. I saw them for awhile, but now my "sent" folder (still visible) is empty. Why are the sent message copies disappearing? I'm a little freaked out...

If you have any Anti-Virus software running then suggest you do not allow access to any Thunderbird files or folders.

Do not allow programs like CCleaner to have any access to Thunderbird files or folders.

At this point do not compact any folders neither manually nor automatically.

Is this a POP or IMAP mail account ? Do you have any backups of profile folders ?

Toad-Hall, thank you SO much for responding. I still have the problem, but weirdly last night, I began seeing, at the bottom of my screen, "Downloading 55000 messages in sent folder." One by one by one. So ok, the folder was way too big and I'm an idiot to have let it get that big. But I had hope! Maybe they would download! I went to bed last night, exhausted but less depressed, about 500 messages short of a complete download. (None of the messages were popping up, but they were "there" somewhere.)

This morning (my computer having gone to sleep, evidently), zilch. No downloading, no sent messages. Sent messages from yesterday (all my test messages) are gone and my "sent" folder is empty again (but still in existence).

Your other points: IMAP, no antivirus software, and -- damn it, yes, should have waited to hear back from Tbird community but tried compacting last night. Everything compacted fine, with message "cannot compact sent folder since it's running another operation" -- which was the downloading mentioned above.

Any thoughts at this point? THANK YOU, THANK YOU for your time!

Forgot to say I don't know what a "backup of profile folder" is?

IMAP emails are stored on the server. Imap accounts synchronise with server to show you a virtual view of what is on server.

I'm presuming you are downloading full emails and not just headers. AS there is a lot of emails in that server 'Sent' folder, suggest you try splitting them.

Logon to your webmail account via a browser to see server folders and emails. Create a folder call it something like 'OldSent' Move emails older than a couple of years, say about half of your sent emails into that 'OldSent' folder.

In Thunderbird Make sure you have subscribed to see the server 'Sent' folder and the new 'OldSent' folder. Please note the server may call the folder 'Sent' or 'Sent Items' or 'Sent Mail' etc So do this:

• Right click on imap mail account name in Folder Pane and select 'Subscribe'
• click on 'Refresh'
• Select the 'Sent' folder and select the 'OldSent' folder.
• click on 'Subscribe'
• click on 'OK'

• Right click on imap 'Sent' folder and select 'Properties'
• select 'Repair Folder'
• click on 'OK'

Now check account settings:

• Right click on imap mail account name in Folder Pane and select 'Settings'
• Select 'Place a copy in'
• Select 'Other' and select the imap sent folder on that imap mail account'

• select 'Synchronisation & Storage'
• select 'keep messages for this account on this computer'
• click on 'Advanced'

Any folder that has a checkbox selected will download full emails.

• If you want 'Sent' and 'OldSent' to download full emails make sure they are selected.
• click on 'OK'

• click on 'OK'

Exit Thunderbird. Wait a few moments for background processes to complete. Change settings on computer to stop it going to sleep.

Start Thunderbird.

Allow folders time to synchronise.

Go into 'offline' mode to check you really have downloaded emails. 'File' > 'Offline' > 'Download/Sync Now' Wait for all folders to be checked with server and a complete copy is on your computer.

If you select some random emails in your various folders and can read emails, you know you have a full copy downloaded.

How to create backup of all downloaded emails: After all emails have downloaded.

• 'Help' > 'TRoubleshooting Information'
• click on 'Open Folder'

A window opens showing you the contents of your 'profile name' folder.

• Exit Thunderbird.

The 'profile name' folder usually called something like 'xxxxxxxx.default' where the x's are letters and numbers and it is in the 'Profiles' folder. The 'Profiles' folder is in the 'Thunderbird' folder. So you need to go up the directory to locate it. Copy the 'Thunderbird' folder and paste into an external harddrive or usb storage of suitable size.

Wow. OK. Thank you. I'll give all this a shot shortly. THANK YOU! You are awesome. I really, really, really appreciate your time and expertise, Toad hall. Will get back to you re if it worked. Hopefully I didn't screw things up by compacting...

RE: any folders that are not displaying in the 'Subscribe' list.

• Right click on imap mail account name in Folder Pane and select 'Settings'
• Select 'Server Settings'
• click on 'Advanced'
• uncheck 'Show only subscribed folders'
• click on 'OK'
• click on 'OK'

Exit and Restart Thunderbird Try again to subscribe to see folders.