I suggest copying the folders from within File Explorer on iinet account and place in Local Folders folder on the PC with MS365 while Thunderbird is NOT running. That places them outside the account, but still saved.

Thank you, David, for your prompt reply. Further investigation since my last post has uncovered a copy of the same email folders on the PC with MS365, so I can put the iiNet laptop aside from the issue before me. But my objective is to get those folders set up so I can see them from WITHIN TB, not outside it - if that is possible. That would give me ready access to saved messages that I am likely to refer to in the future. The mail folders are located at D:\Thunderbird\sp5uno6a.default\Mail\exchange.iinet.net.au. I assume they were written there when I was using iiNet for email on this PC. There are two folders for each subject, one an MSF file (which could be an index) and the much more substantial File proper, which I assume contains the saved messages. The most recent activity on folders in this directory corresponds with the date I switched to MS365, confirming that these are the folders I am after. The Account Setting currently on TB for Message Storage shows the Local Directory at D:\Thunderbird\Local Folders. Although I have inserted manually in TB folders with the same names as those saved earlier, and a few of them now have one or two messages stored in them, I cannot find where they are being saved. I'm guessing that would be where I might save copies of the folders saved earlier. But mine is an uneducated guess. Any advice that will enable me to copy those email folders to where I could again access them from within TB would be greatly appreciated.

My suggestion in earlier response still applies. Copy both entries for each folder, e.g., INBOX, INBOX.msf.

Your earlier suggestion places the mail Folders outside the account, if I read you correctly. My objective is to place them somewhere inside TB so I can at least refer to them there, preferably in the same account. I have now confirmed that TB is writing the "new" mailboxes (ie mailboxes entered into the single account on this TB installation) at C:\Users\UserName\AppData\Roaming\Thunderbird\Profiles\sp5uno6a.default\Imapmail\outlook.office365.com. As an experiment, with TB closed I copied from my D drive (ie archive location) to the above address the mail files Amazon (2,802 KB) and Amazon.msf (65 KB), which were saved from a previous account. Now when I open TB and select the folder Amazon, I initially see all the messages stored in this box on my D drive. After a very short time, they all disappear. When I check back using Windows Explorer, I find that the file Amazon.msf has been over-written and is now just 4 KB in size. What is going on here, and how can I overcome it, please?

My suggestion to place in Local Folders was to protect you from that. Although I don't know the mechanics as well as some on this forum, IMAP accounts sync to the server, not to the PC. Since those messages are not on the server, they are removed. It is possible to create the folder online, and then to copy from PC to online folder, but that is a problem-prone process that doesn't work well (I say that after having read of the attempts by others on the forum), but you may want to do that. If you do, I suggest doing only a small percentage at a time.

Thanks again, David. Following your advice, and copied the files to the Local Folders location with TB turned off. Interestingly, most of the files did not complete the pasting process on the first attempt. When I fired up TB and looked in Local Folders, they were not there. But after I pasted a second time (which generated the progress marker) they were all good. I turned TB back on, and they are all there in local folders. I also tested opening one of the archived messages and sending to my own email address a message in response, and that worked. With your kind assistance, I have achieved exactly what I sought. Again, many thanks.