@jack I thougt only Gmail uses non-standard IMAP flags but most clients support them. Have you tried Spark? I installed it today on all my devices and try to getting used to it.
@renem I tried Spark a while back but don't recall why I didn't stick with it. Maybe worth another go? (Although I'm not terribly unhappy with Mail.app these days)
@jack I'm also quite happy with the native apps. But I was curious about the latest additions to Spark, so I installed it again. I'm unsure if I can stick to it, but I'll give it a try for at least a few days.