I would like to filter unflagged emails, but this is not possible now
Yes, you are right but you can create custom search folder which will show you unflagged emails. And flagged mails are available in To-Do section of right sidebar.
Hello Gabriel, thank you for answering
I have found the flagged mails in the To-Do section of right sidebar.
But how can I create a custom search folder which will show me unflagged emails?
as my college Gabriel wrote above you need to use advanced search.
Go to Main menu --> Edit --> Find and advanced search dialog opens. Choose folder where you want to search. In the row where is chosen “Flag” choose “No” and click “Create Search Folder”. Name the folder and click OK. Folder with found emails displays in Global Folders under section Search Folders.