Updating folders thunderbird
If Thunderbird doesn't recognize that a folder contains unread messages try setting use_status_for_biff true.It causes Thunderbird to explicitly select each folder to update the message summaries, rather than using the STATUS command.But you can configure Thunderbird to automatically check for new messages in other folders.There are two ways to do this but adding more folders may affect Thunderbird and/or server performance: If you are the admin for the IMAP server you may be able to configure it to check for new mail in all folders and notify the email client using the IDLE command.Sometimes, the folders in Mozilla Thunderbird lose track of the underlying structure—messages actually present are not shown, or deleted emails are still present.Thunderbird can rebuild the folder index, which displays the message list quicker than when the folder's full contents are loaded, and make it accurately reflect the messages you have in the folder.It can test folders across multiple accounts, and is displayed with a special icon in the folder pane.
You could also create a virtual folder using saved search that tests [Status] [is] [New] for all of the folders you're interested in.It might make it easier to see what fodlers have new mail.If you don't like how Thunderbird indicates a folder has new mail you could add something like the following to a user file in the chrome directory in the profile to make it easier to see which folders have new mail.Note: When subscribing to filtered mail folders such as Spam or Junk, all the mail coming into those folders on the server also will be downloaded and synced to your selected local mail client as well.
If you are using a metered Internet connection or have limited bandwidth, please be aware that the transfer of email does count toward your data usage.
Any of those messages are downloaded to the inbox and a special header added so that a message filter can test what folder they came from.