Folders containing New emails marked blue and yellow star.

If auto filtered upon download to Inbox then they show up as new, blue and star no matter what folder they get auto filtered into.

But what is regarded as 'new': New means new since you last selected a folder.

Senario : you have new in Inbox - Inbox is blue, starred, so you select Inbox folder - emails that are new have star - maybe you decide not to read any emails meaning emails still have star at this point. Then you select another folder. The Inbox is no longer containing anything 'new' because nothing has been downloaded since you last selected Inbox.

Senario: This time when you select to run filter on Inbox - it filters all relevant emails in Inbox, but in addition a new mail arrived during the process because you have set the check emails every minute which is hardly any time since last check, so it downloaded and got moved instantly. You would not have seen that email in the Inbox. That would trigger the folder which got the new download to become blue with new.

Whilst you can set up Message filters to only filter upon manual selection, it is usual to also have it set up to do it automatically. I've got both selections set up in Message Filters as it does no harm.

If You have Message Filters automatically filtering upon download, then folders and emails get the 'new' flag; blue and star no matter what folder they get filtered into. But they only remain new and blue until you select the folder and stay in folder. You move away from folder then no longer new even if you did not read them.

When it comes to imap accounts, it may be worth checking to see if the folders are designed to auto check for new mail. Maybe you have new mail in folder but you would not know it because folder is not set up to auto check. Meaning when you select a folder it auto synchronises, downloaded new mail and shows this.

Right click on folder and select 'Properties' Is this checkbox selected : 'When getting new messages for this account, always check this folder'.

But in my case it doesn't work that way, at least not always.

Scenario which I had today: I have Inbox activated and 2 new emails in it - inbox is blue with stars. The rest of directories is black, without stars. I run filters on Inbox. Emails disappear from Inbox - Inbox become black, no star. The rest of directories is still black, without stars as their were. I change the directory, click this one in which I know the email should be. The directory becomes blue, and star appear!

So, if these emails are new in the folder, why the folder change colour and get star only when I click this folder? How do I know that I received new email?

Try something higher than one minute. Suggest 5.

It still doesn't work properly. Nothing change.

Today's scenario: All folders are black - looks like no new messages at all. But I expected a few, so I checked email by www client. There are emails on the server and they even are in target subfolders (they were moved from inbox). So thunderbird downloaded them, run filters, but "forgot" to mark folders. Furthermore... I have one saved search folder which should show all emails from last month and there are no new messages there. What is interesting, the number on the badge next to the folder name shows correct number of unread emails, so Thunderbird counted this email, but it is omitted by search engine and is not treated as new email.

And at the end, as usually, when I click the folder it becomes blue with star (the number of unread emails doesn't change), and in the same time my save search folder also changes colour.