What is your anti-virus software?

Sorry for being tardy. Real life intrudes.

My "technical" suggestion is to manually delete the index files for that folder. It essentially accomplishes the same thing that repairing the folder does but slightly differently. Here's how:

- Right-click on the folder name and choose "Properties" from the context menu. Make a note of the Location. If it's an IMAP account it will look something like

imap://(the account user name)@(the account server name)/(local folder name)

If it's a POP3 account it will look like:

mailbox://(the account user name)@(the local folder name)/(the local folder file name)

- In Thunderbird, go to <Help | Troubleshooting Information> . Scroll down to "Profile Folder" and click the "Open Folder" button. This will open your system file browser inside your Thunderbird Profile.

- Close down Thunderbird.

- If it's an IMAP account, navigate to the folder ImapMail\(the account in question, as found in the Folder Properties above).

If it's a POP3 account, navigate to the folder Mail\(the account in question, as found in the Folder Properties above).

Here you will see files representing your various folders. There will be a pair for each folder, one with no extension, the other with a .MSF extension. The MSF ones are the indexes.

- Delete the MSF file for that folder *ONLY* - be careful to not delete any others! The MSF index file will be recreated when you next start Thunderbird. Your mixed-up headers should be correct again.

This whole business only corrects the symptom. That the indexes keep getting corrupted is the real problem. If this persists, it's a sign that there's something else going on. The most likely cause is your Anti-virus, as was mentioned by christ1 . It could also be general computer problems, such as a failing hard disc or file system corruption, although I would expect you would be having other computer problems besides just this.