So, it turns out, if I go to the Sent Items properties and repair, it deletes all the email in it except for September's email. I can copy in an old backup of the sent items and they'll be back, but I still can't copy the emails. If I click repair, it all goes away again. If I look at the actual folder Sent Items.mozmsgs all 9000+ emails are in there, so what's going on? What can I do to prevent this from happening?

Thanks!

Okay....further investigation. It turns out my backup of my TB install also has a corrupted sent items folder. It's only 98MB. Luckily I had just moved to a new PC and still had the old PC and copied the sent items from there which was 1.8GB. Unfortunately, I appear to be missing a few emails right at the beginning of September that the old PC didn't have (since I had switched PCs) and the new install had corrupted and I can't retrieve.

The scary thing is I have no idea if this has happened to other folders and I just don't realize it yet. The emails APPEAR to be there until something like this happens.

Any advice is appreciated.

re :If I look at the actual folder Sent Items.mozmsgs

Sent Items.mozmsgs. This is a folder created so that Windows Search can search for emails.

It is not the mbox file. It is not the file storing emails that Thunderbird uses to display. If you synchronise your subscribed folders to get a copy stored on the computer in your Thunderbird Profile folder, then emails are stored in an mbox file. 'Sent Items' should be a file with no extension in the mail account folder. There should also be a 'Sent Items.msf' file which is an indexing file - does not contain emails. Thunderbird will create one if it does not exist.

re if I go to the Sent Items properties and repair, it deletes all the email in it except for September's email. Repairing the index file (Sent Items.msf) means that it checks the mbox file and displays what is in the mbox file. The mbox file is synchronised with server. So it would seem the only emails on the server are September.

Suggest you try this. Close thunderbird. As you seem familair with Profile folders.... Put a copy of the 'Sent Items' mbox file into 'Local Folders' mail account which is in the 'Mail' folder.

Restart Thunderbird. Go into 'Offline' mode. 'Sent Items' should appear in the 'Local Folders' account. click on 'Sent Items' folder. Right click on one email and use 'Copy to' the imap mail account 'Sent Items' folder.

Go back into 'online' mode.

Double check: Logon to webmail account using a browser - check is that email now displaying in the 'Sent Items' folder? If yes, then repeat procedure. Go offline, copy email to imap folder, go online mode.

When you are happy that all emails are back on the server, you can delete all the emails in the 'Local Folders' -'Sent Items' folder.

The sent mail isn't tied to a server as this is/was a POP set up, not IMAP. It wasn't being synced with a server folder. The IMAP server was only the GMail server I was/am trying to move to. The original Sent folder was local only.

The dates are such that I think installing Windows 10 over Windows 7 is what corrupted the Sent Mail folder/file. I did that on September 3rd and only email from then was available. My last good Sent file was on my old machine. Unfortunately it was only up to Aug 28th. So, I lost the days between then and September 3rd. :-(

Can't really think of any way to get them back.

re : The original pop mail account

Did the old pop mail account download from the same gmail email address mail account which you now use as an imap mail account? If yes, then the emails will be in the backup pop mail account 'Sent' mbox file - no extension. Do you have that file? If yes, put that 'Sent' mbox file into 'Local Folders' and then do the same process to get emails back on server.

If the old pop mail account was not downloading from the gmail email address/account, but another email address, then you cannot put the emails back onto the gmail server. In which case, just leave them in the 'Sent' Local folders; at least you would have a copy in thunderbird.

No, I'm moving my personal email address, which was hosted elsewhere (downloaded using POP) to Gmail (using IMAP). The email address is the same, just the server and protocol are changing. So, it's the original Sent folder from the POP account prior to any migration to Gmail that was messed up by upgrading to Windows 10. In actuality, it has nothing to do with Gmail other than the migration to Gmail is what made me aware of the corruption.