I use actually Profimail, that has this feature.
Why I need this?
Because I hide messages that I want later answer on my desktop pc. But unfortunatly all hidden messages were implicitly marked es read.
Additional two feature for hiding would be nice(also working with Profimail)
- a sync of all hidden messages between all mobile clients
- temporary unhide. Needed for geting an hidden mail but I don't wont to unhide all for this.