If the Message List is displaying a list of emails, but when you click on one of them, there is no content, it means the 'index' file used to created the Message List is out of synch with what is actually stored in the mbox text files (they contain downloaded emails) or whatever is stored on server if only headers where downloaded.

When you click on 'Repair folder' button, it forces a rebuild of the index file, so it matches whatever is in mbox file or on server.

Using the 'Repair Folder' is helpful if:

• Emails in list shows one particular subject but when selected, wrong email is displayed in Message Pane.
• Email in list has already been deleted, but is still showing in the index file, so it appears in Message List but as email does not exist, it appears blank in Message Pane.
• The Message List seems to be missing emails.

'Repair folder' does not delete emails. It rebuilds the index file.

Imap accounts should only display whatever is on server. Do emails exist on the server as viewed via webmail account or is Thunderbird imap folder showing exactly what is on server ?

Emails can be removed from the server by...for example:

• Logon to webmail account via a browser and delete emails.
• Access mail account on server via another device using imap and delete emails.
• Access mail account on server via another device which is using Pop and it is not set up to 'Leave messages on server'.

Or removed by your Anti-Virus product: Anti-Virus products who are permitted to 'fix' or 'auto fix' a Thunderbird mbox file when it finds something it does not like, but ends up causing mass deletion because it fails to understand that the one file contains more than one email. Ideally, if you use an Anti-Virus product do not allow any auto fix and never allow a fix if it relates to anything in your Thunderbird profile. This can often be the culprit if the index file thinks emails should exist but they do not because the Anti-Virus does not look at the index file as it does not contain emails, so it's left alone.

Note: regarding 'Spam' and 'Trash' folders. Most servers will auto empty these folders periodically. Emails that have been in those folders for more than X days get removed off server; deletion dates vary from a few days to 30 days.

Do you have a recent backup of your Thunderbird profile ?

I might have a recent backup. Thank you.