That was unusual to have a Pop account Sent folder suddenly go missing.

The index file (called 'Sent.msf') had not been effected because it still had the list, but when you selected to open an email, the contents were missing because they did not exist in the 'Sent' mbox file that actually stores the emails.

The 'Repair folder' button repairs the index file and forces it to check on what exists in the mbox file. As the emails did not exist then the rewrite of index file does not find emails, so 'Sent' appears empty.

Repairing the folder did not delete the emaisl; they were already gone.

Why? Good question!

Possibilities: Anti-Virus was allowed to scan the file - it did not like something and chose to try and fix it. Anti-Virus products do not understand that an mbox file is not just one email, it may contain eg: 47000 emails all in a single text file. That would explain why it went missing. It would also explain why th eindex file was left alone.

Advise; Do not allow any Anti-Virus or file cleanup software any access to your Thunderbird profile folders as they are well known to cause havoc.

Do not store large numbers of emails in a single folder because in reality they are being stored in a single text document which is a bit risky if things go wrong. It could be harder to compact folders if they are too large; compacting removes all traces of previously moved and deleted emails thus helping to reduce file size and maintain a healthy file. It is much harder to open very large text files should you ever need to try and rescue emails; in some cases it can crash a computer due to the excessive use of RAM. Suggest each year you 'Archive' the sent emails. In Account Settings > 'Copies & Folders' - Set up your 'Archive options' to use 'yearly archived folders' and also select the option to 'keep existing folder structure of archived messages'. Then select emails and click on 'Archive'.

If you have a backup then let me know and I'll advise on how to get it put back into the pop account.

This is how to check on your current profile folders to see if the 'Sent' exists somewhere in the profile.

• Menu icon > Help > More Troubleshooting Information
• Application Basics section - Profile folders - click on 'Open Folder'

• click on 'Mail' folder

Do you have several pop accounts ? Is one a duplicate? Select pop account folder (check both pop accounts if more than exists) the 'Sent' no extension file is the mbox file that should have emails. But do you have another eg: 'Sent-1' ? or 'Sent.bak' ? If yes, do they have a good size ? It would help to post an image showing what you see.

re Or should I worry about it in other folders?

If you stop AV accessing profile that will help. If you setup Archiving Options as advised and perform the Archive action on older emails (do it in batches) then it reduces the size of files, so all eggs are not in one basket. Do compact folders after archiving so that old traces of moved emails are cleaned up from file thus reducing it's size and helping to maintain a healthy file. Regulary compact Inbox, Drafts and Junk as those folders get the most activity of moved or deleted emails.

Thankyou for this helpful answer - especially knowing the difference between the .msf and the .txt file.

I do have a backup - just a simple entire profile folder I back up every couple of days. Is there a way to import just the "sent" .txt and .msf from that into the current profile? can I just delete the existing .txt and .msf sent files and drag back ones in? I would only lose a week of outgoing sent emails so that is not big deal.

And, I just realized - I think this happened when I was trying to pull some of the emails from past years into a backup folder I just name "backup'. But from what you are saying I should be using the archive feature. I avoided the archive method because I understood I would have to restore each time I wanted to do a search of archived messages.

The only AV software I have is Windows 10 - and I just excluded my moz. profile folder.

I use local folders, so all of my po3 go in the same folder called "LocalFolders"

Let me know how I can import my sent file - if I can just drag and drop it into my profile mail folder. Thanks!

The second image shows contents of what I believe to be your 'Local Folders' account.

The 'Sent' mbox file dated 5/8/2023 12:06 PM has a size of 10,426 KB So that file appears to have emails in it.

Is this image showing the current 'Local Folders' or the backup 'Local Folders' ?

If it is the backup:

In Thunderbird

• Menu icon > Help > More Troubleshooting Information
• Application Basics section - Profile folders - click on 'Open Folder'

• EXIT Thunderbird now.
• click on 'Mail' folder
• click on 'Local Folders' folder

You will see a 'Sent' (no extension) file and it is likely to have a very small size because you say the 'Sent' folder has lost all it's emails. If that is true then you can delete the 'Sent' and the 'Sent.msf' files as they are of no use OR rename them to have a .bak extension eg: 'Sent.bak' and 'Sent.msf.bak'.

Then get a copy of the backup 'Sent' (no extention) file Paste it into the 'Local Folders' folder - thus replacing the one you deleted.

Start Thunderbird.

re:. I avoided the archive method because I understood I would have to restore each time I wanted to do a search of archived messages.

No, you do not need to restore anything. The 'Archives' folder is just another folder in the Local Folders mail account. The 'Archive Options' set up using yearly folders means emails get sorted and stored based on the year. Keeping existing folders means if you have a folder for year 2022, then it will also contain a 'Sent' folder of archived sent emails. So making it easier to find and sort emails.

You can tell the Global Search to include the Archives folder... Right click on 'Archives' folder and select 'Properties select checkbox 'Include messages in this folder in Global Search results'

You can search independently on any selected folder using the Quick Filter search or use the Global Search to search all folders or even use the 'Menu icon ' > 'Find' > 'Search Messages' (or if using the top 'Menu Bar' toolbar: 'Edit' > 'Find' > 'Search Messages' ) search method.

Thank you. That worked perfect - and I was glad it did not disturb any of my existing folders. I will not set up the archiving. Thank you for your help!