It has tools (add-ons, to be precise) that allow messages to be exported.

I'd alert and warn you about the "Archive" feature built into Thunderbird. Whilst it does remove older messages from working folders, (keeping folders and message stores down to reasonable sizes) the archived messages are still stored within Thunderbird, so I'd recommend the export route for fully independent storage.

This add-on: https://nic-nac-project.org/~kaosmos/mboximport-en.html greatly simplifies the process of mass exporting. There are also built in filter tools and add-on enhancements to the filters that could be used to auto-save messages outside Thunderbird.

Thanks Zenos for your quick reply. When you say the 'archive' feature removes older messages do you mean when it is being used as an IMAP client (ie emails stored on the web), or does it also delete from the folders on my PC if I use it as a POP client (ie emails stored locally on my PC), which I believe I can do. (please excuse my phraseology if it is incorrect).

I have taken a quick look at the add-on and it looks as though I could export the important emails to an alternative file format for safe keeping, so I think I'll give it a try in parallel with my Live Mail rubbish until I am happy.

By default, the Archives folder is within the account it belongs to. So in a POP-connected account, the Archive would be on your local storage device. In an IMAP-connected account, its archive folders would be on the server, but these can be arranged to live on your HDD.

You can use Thunderbird with either POP or IMAP, assuming that your mail provider offers both.

The Archive system is more a housekeeping function, since it doesn't truly separate out the messages from Thunderbird's mail store. It separates older messages from your current, working, folders, making Thunderbird a little more responsive and IMHO easier to manage. I don't need to see last year's messages, but they are there, to hand, and can be searched if appropriate.

Given your experience where a mail client has trashed its store, it would be prudent to save messages outside the client and this is where the suggestion for ImportExportTools may be useful to you. This will let you store messages in a free-standing format and thus isolated from the mail client.

Thunderbird has a somewhat unusual arrangement that uses a special account, named Local Folders, which serves as a local store on your own storage device and is independent of any email servers. It serves as a useful place to keep offline backups of messages away from IMAP servers. It also hosts RSS feeds, newsgroups, and an Outbox for messages waiting to be sent.