In the message above, when I wrote "A little while ago..", I meant "An hour or so ago...".

I guess this is all part of a larger problem I am having - detailed in a separate thread "Cannot empty Deleted folder and Thunderbird is creating new folders", which I am having little success in tackling. But the problem in this thread is urgent and can probably be resolved sperately from the other problem, so I have started a new thread for it.

This set of instructions from sfhowes should help you: https://support.mozilla.org/en-US/questions/1427799#answer-1611744

Look for the mbox files in the old profile, in Mail/<popservername>, ImapMail/<imapservername> and Mail/Local Folders, and in sbd subdirectories. They are the large files with no extension, named after folders, e.g. Inbox, Sent, Archive... Each mbox has a .msf index file.

Copy the mbox files, renaming if necessary, to Mail/Local Folders in the current profile, while TB is closed (ignore the .msf files). Restart TB, and you should see the folders under Local Folders in the Folder Pane.

Thanks - this looks useful. One question however - In the "Mail" directory I seem to have a mail.plus.net directory dated today (together with mail.plus.net.sbd) and a mail.plus-1.net directory dated yesterday (together with a mail.plus-1.net.sbd). Is this significant? I assume I should take the Inbox from the mail.plus.net directory.

I have figured out the above question. mail.plus-1.net are the files for a secondary email address that I have. Unfortunately is appears that at the time when my back-up was done (at 3.00am this morning), the damage had already been done and the missing emails are not in the back-up file either. It seems I have lost 6 months worth of emails from the first six months of this year. But even worse than that, I have no idea how this happened and what is going on - so I may well lose even more. This is scary. I have explained what is happening in the thread "Cannot empty Deleted folder and Thunderbird is creating new folders". No-one has come up with an explanation or a resolution.

Hopefully you can get some more assistance in the other thread. Honestly, Thunderbird is not my expertise but I really am passionate about helping people recover from data loss. If it can even be done at all.

It should be doable though because you smartly took the time to backup your Thunderbird profile which many people never do. Then they come here desperately begging for help to recover their lost emails. Which is a horrible thing to see & help with because data loss is very painful. Especially if you have no backups of the data.

When databases in Thunderbird and other programs get that big (20GB+), they can develop some corruption. And it can be very hard to repair it without losing some emails. If you did enough backups over the years, I'm sure the majority of your emails are ok. But the recent ones might not be. How often do you backup the Thunderbird profile?

Off-topic thoughts that could maybe help you in the future: Maybe Thunderbird autobacking up the files containing the emails could be a good feature to add. But the problem is those mbox files can get quite MASSIVE. And backing them up frequently can cause people to run out of hard drive space quickly causing out of space errors and could wear out SSDs faster with too many writes to the hard drive. Large mbox files can also slow down Thunderbird's opening time & take a long time when doing email searches.

My question to you is a bit deeper on a personal level so I'd like you to really think about it: Are those emails you lost really that important? Or were you ever going to read them all in the future?

This style of thinking can help soften the blow.

I'm not saying this to anger you but personally I came to this realization years ago when I used to obsessively download a lot of email with Thunderbird to archive it for future reading. I honestly only looked back at my old emails a handful of times for information I had forgotten or to find a good memory. Although if those emails contained family/personal photos, I would definitely have gone back and looked at them more often. But they did not so I never checked it often. Eventually I moved computers and never looked back at those archived emails ever again.

So I'm wondering are these personal or work emails? Because when you get a lot of email it can grow to overwhelming numbers and be extremely hard to manage even if you backup it up religiously. 20GB+ is a lot! But maybe some of that size is due to photos or attachments. I'm just amazed that you're receiving so much email. Maybe there's something you can do to minimize the amount of email you have to save. Or perhaps delete more of it from the webmail accounts before its saved locally so that corruption of the mbox database is less likely.

My friend told me to try to treat some data as "ephemeral". Definition: Lasting a very short time; short-lived; transitory

It took a long time for me to treat data as ephemeral. I do that for emails now. But doing it for other data like photos and videos is very hard. Anyway, that's all to say that you're not alone in losing data and dealing with the extreme difficulty of preserving & recovering it. Even when taking great care to make backups. Which is something everyone should do with important data.

Thank you for your long and thoughtful post. I have a scheduled back up overnight every night - but of course each backup overwrites the previous backup. And it looks like I cannot get back far enough to recover the 5 months of emails I have lost (1 Jan - 1 Jun 2024). When I first looked at my TB yesterday morning, I saw that the number of emails in my Inbox was too low (about 2,000) but when I clicked on "Properties" on the Inbox, the number of emails it was showing was about right (11,000), so I thought that by clicking the "Restore", it would reindex the whole 11,000. Instead it deleted 9000 and just kept the the 2000. I keep all my emails from this year in my Inbox and Archive all previous years.

With regard to your other comments, it was precisely because I realized I had accumulated too many emails and was trying to go through my Archive files (both under the Inbox and in my Local Folders from previous software I was using - Turnpike) and delete unwanted emails that I have got into my present mess. TB just started to go haywire and I got the crazy behaviour I have described in my other thread "Cannot empty Deleted folder and Thunderbird is creating new folders". I was gradually getting things under control and managing to delete folders that kept popping up with either garbage or duplicates in them. So that by the evening of Tuesday, I thought I had everything under control. Then I woke up on Wednesday to find I had lost these 5 months of data.

You asked whether the emails I had lost were really that important. Well probably many of them were not but I do frequently do searches on my old Archived messages for information that I need for something I am doing currently. I have however unsubscribed in the last few weeks from a lot of sources of unnecessary incoming emails and I have pruned the emails in my Inbox and Archives of much "ephemeral" material - I reduced most of my Archive files by 30-40%.

You mention the Thunderbird Backup. If you mean the Export feature under Tools, then it says when you click on it that if your profile is more that 2GB you should backup yourself - which is what I do. My archive of emails goes back to 1996 which is why it is 20GB+ - and also why I have put off pruning it until a week ago or so. It takes a lot of time to go through and prune that much material.

Anyway it looks as though it is only that 5 months of emails that I have lost. However, as I still have no idea how I lost these emails (I have had no useful responses on the other thread), I am worried that I may wake up to another big loss. So I have downloaded a programme called Mailstore, which I hope will give me some additional protection by providing an alternative backup.

Thank you again for your input.