Fastmail, a powerful yet polished email service, adds a unique perk to flagging/starring messages.
Instead of hoping that “pinned” messages stand out with an icon in the message list, you can re-sort the list so pinned messages sit at the top of all others, regardless of their age.
If your inbox and folders double as a task list, at least for some things, this is a great way to prevent those tasks from falling out of your inbox and off your radar.