Emails can infact disappear from Thunderbird, permanently too, for any number of reasons. Messages are stored in mailbox (mbox) files by default. The Search function uses the Global Message database to present the search results you see. This database may contain references to messages that no longer exist in any mailbox (mbox) files. See https://support.mozilla.org/en-US/kb/rebuilding-global-database for details. Also see https://support.mozilla.org/en-US/questions/1348725 for related answers.

Thanks for reply but I still don't understand the storage of T/Bird emails. It's frustrating that so often when I want to find a sent or trashed email I can't find it. Sometimes a search will unearth it - but often not, and then it's not actually in the folder to which the search directs you. How do I ensure that I don't "lose" emails? I would expect to find them in All Mail, but this is not the case.

Go to Profiles - Where Thunderbird stores your messages and other user data and read through that article as many times as it takes you to understand its contents. The mailbox files I was talking about are all found in your Thunderbird profile folder. If you've read and understood the knowledge base article linked above, then you should know what I'm referring to when I write profile folder. Likewise, the files used to store the Global Database are also found in your Thunderbird profile folder, as explained in the article I linked to in my first reply.

The only way you can prevent loss of Thunderbird data (emails, e.t.c), is to have a backup of that data. There's just no substitute/alternative for data backup. Data that you do not have a backup of, is data you are willing to lose.

Who is your email service provider?

My em provider is Gmail Have read through your suggested articles I think I understand, but find them very complex. I'm just on 80 so understanding technology does not come easily. It would seem that to ensure that I don't potentially lose emails I need to create a separate back up folder and copy them daily. This would seem to eventually create a folder so large it would become unwieldy ineasily access a particular email. I like Thunderbird but I'm not sure that another platform such as the Gmail platform would be better suited. Do other platforms also have this issue where, as you say, emails data can be lost?

My em provider is Gmail Have read through your suggested articles... I... find them very complex.

I thought you would find them very complex, which is why I didn't directly respond to your original question of, "Can you advise me just how the email storage system works?"

I'm just on 80 so understanding technology does not come easily.

The good thing is that you don't need to have a deep understanding of technology to make good use of it. You therefore don't need to know a whole lot about how Thunderbird's email storage system works (leave that to its developers and engineers). It should be enough to know that;

1. Thunderbird stores messages in mbox files 2. It also maintains an index of messages, known as the Global database, used by the Search feature. 3. The mbox files contain the actual messages, while 4. The Global database is just an index (or catalogue) of the messages stored in the mbox files

When you perform a search in Thunderbird, it's the Global database (index of messages) that facilitates the results you see. When you perform a search, the mbox files which contain your messages, are not accessed directly to find the messages that match your search terms. If that was the case, searching would have taken extremely long to return the search results because of how big the mbox files tend to become. In fact, the mbox files are the largest files you will find in your Thunderbird profile folder. Instead of reproducing details about the Global database on this post, you can re-read all about it here Rebuilding the Global Database. It's the same article I linked to in my first reply. It states why you may want to rebuild the Global database. While Thunderbird generally does a good job of keeping that database updated, so that it only contains references to messages that actually still exist in the mbox file, sometimes it falls behind and ends up with references to messages that have since been deleted from the mbox files. This causes the scenario you're facing now, whereby the search results include the message you're looking for, but that message no longer exists in the location pointed to by the database. This can mislead you into thinking the message is not really lost/gone when it is in fact gone from the message storage files (files).

It would seem that to ensure that I don't potentially lose emails I need to create a separate back up folder and copy them daily. This would seem to eventually create a folder so large it would become unwieldy ineasily access a particular email. I like Thunderbird but I'm not sure that another platform such as the Gmail platform would be better suited. Do other platforms also have this issue where, as you say, emails data can be lost?

There's always the risk of losing data, regardless of platform, and the only way you can ensure against loss, is to have a backup copy of that data. The level of risk differs from platform to platform. There are many ways you can approach the issue of data backup. It eventually boils down to what works for you economically and practically. The simplest backup strategy you can arguably have is that daily copy, but it doesn't have to be that tedious.

There are other steps that you can take to minimize risk of Thunderbird's data loss, and key among them is to make sure your security program (antivirus software) does not interfere with Thunderbird's filesystem operations. You can do this by adding your Thunderbird profile folder to the antivirus' list of exclusions. There is a long history of antivirus programs causing havoc with Thunderbird's normal operations, and that includes deleting entire mbox files (and all messages contained therein), when a scan finds or suspects an infected message inside that mbox file! However, before we veer way off target, first confirm if that lost message is also gone from Gmail platform. See https://support.google.com/mail/answer/7015314?hl=en-GB&co=GENIE.Platform%3DDesktop for details. Emails should not be disappearing from both Thunderbird and Gmail.