I solved this problem on my PC. The first thing to try is to right click on the folder where the messages have been lost. Then you will find a menu that includes properties. Left click on properties and an option to repair the folder appears. I tried this but it did not work. How I fixed the problem was as follows: This applies to a PC but I would imagine that a Mac uses a similar storage technique. What I did was to navigate tot he file location where Thunderbird stores messages and then I opened the large inbox file using a text editor. I used MS notebook. I could then see the HTML content of each message. As there were so many messages, it took a long time to load them up in MS Notebook. Any simple text editor would do. In the headers of each message was a line giving the X-Mozilla-Status:. In just about every case it was showing it as either 0009 or 0019 or 001b. X-Mozilla-Status: 0009 or X-Mozilla-Status: 0019 or X-Mozilla-Status: 001b I then used the search and replace function in the text editor to change the X-Mozilla-Status: to 0000 for every message. This step took a long time. X-Mozilla-Status: 0000 I then over wrote the inbox file with the modified version (having previously backed up the corrupted inbox file) which had the X-Mozilla-Status: lines altered to 0000 for all messages. Rebuilding the index files using the Thunderbird repair function and hey presto, my missing e-mails reappeared new, unread messages, complete with attachments. I was then able to move them to the archive 2016 folder and all's well that ends well. Thanks again for your help and I hope that this does not become necessary again. I have a fairly powerful PC and even so, each stage of this process took a lot of time.