Hi, I suggest you to create an inactive pop3 account so you can replace your msf file (I don't deal with sbd, so I don't know).

I use a Mac but it should be the same on Windows: Create a new mail account that does not exist, or maybe one that you only read online (I tried with vivaldi.net). As you type your e-mail, you may see a clickable label "configure manually" appear. Click on it so you can force "POP3" instead of IMAP. Thunderbird allows the creation of an account even if the datas are not valid. So do not type all the credential and keep the account even if it is not valid - all you need is that the folder structure is created. Then you have a final screen, the server settings, with the path to the storage folder. Head to it and quit Thunderbird. In the folder, paste a copy of your SBD files with their index (keep the originals safe!). Maybe you should rename them as the default files created to lure Thunderbird. Then restart Thunderbird and see if the fake account lists your e-mails.

For long term storage, I would advise to use the archive function on them in a local folder which IMO is the best storage for old e-mails. And get rid of the fake account.

Thanks frederic. I was thinking something like that might be the answer but it seemed pretty technical to attempt so was waiting for a reply here. What I've found that seems to be working is to copy over the pop .msf files and contents of the pop .sbd folders to new .sbd folders that I create under "Local folders" which are just that - local. It's a little tricky but is mostly working. I do the copying in the C:/User directories and then close and open Thunderbird to see what sticks. So far it's working pretty good.