FWIW, with Thunderbird you can easily label email priority with keys 1-9. Where 1 is the highest priority, it will mark the email in red. So, email not read is

FWIW, with Thunderbird you can easily label email priority with keys 1-9. Where 1 is the highest priority, it will mark the email in red. So, email not read is bold, important email not read is bold red. Works pretty well for me as a backlog/todo list.