Close Thunderbird. Open your Enquiries file in a text editor (Notepad or similar). Do a search for X-Mozilla-Status. What numerical value do you see there? There will be many of these entries; one per message.

If it is not zero, then try doing a global search-and-replace to change that value to zero. (Formally, it should be a string of several zeroes. I'd suggest you replace whatever you find with the equivalent number of zero digits, e.g. 0009 changes to 0000.) Save your changes, restart Thunderbird.

Zero indicates an unread message. Certain numeric values indicate a deleted message, and so such messages will not be displayed. By re-setting all the non-zero values to zero we make them all effectively unread so they will re-appear.

Thanks Zenos, I'll give it a go. Currently trying to open Enquiries in Notepad, but it might be too big. I assume you mean, open Notepad, open file, open all files, open "Enquiries"? Can't see how else to do it. And I'll get back to you if this works and I have to save!

I've been using Notepad++ for so long I can't remember what regular Notepad can or can't do. Your 500 or so messages shouldn't be a huge file, but of course it may contain legitimately deleted messages and so have rather more than your expected 500.

You can do this in Wordpad or Word, but you need to take care that it is saved as a text file and that no sneaky ".txt" extensions are added. Actually, the same consideration about an unwanted extension goes for Notepad too.

Try shift+right-click on the file and choose "Open with".

Or, open Notepad, then drag-and-drop Enquires onto it.

I got it to open in Notepad++ and this is what I got (see screen shot). The entire 228mb file is nothing but this "NUL" thing. Think I'm in trouble.

Er, yes, that is an extremely empty file. :-(

You have twice given the file's name as "Enquiries - is this intentional? I take care to avoid "funny" characters in Thunderbird's file and folder names. A folder in Thunderbird is represented by a file in your file system and so needs to observe the same constraints as apply to filenames. For that reason I take care to avoid slashes, asterisks, quotes and the like because they all have special meaning to the file system. Thunderbird is flawed in that it doesn't raise objections to "improper" filenames.

I'm not saying this is a fault, but I don't know how the file system copes with a filename with an unmatched quote. We use quotes formally to specify so-called "long filenames" that include spaces, such as "my house.jpg". My conservative approach would lead me to prefer the name my_house.jpg and then I avoid the complication of the space and the need to quote the filename.