I would like to have a way to show all important emails or email tagged with given tag.

There is currently a "Favorite folder" list, but what I want to track is not in a given folder, but accross all accounts and folders. 

Use-case: I need to remember to answer to a job offer (in my private account), to review a patch on KApidox (in my kde account) and my brother wants to plan the next climbing session (in my private account). It would be great to pin them somehow and find them in a fast manner.