The most powerfull OS of the universe have mail app that do not function for so long time...... How about this Tim !?
I've wondered this too.
I get that there are a thousand obscure features that we can't expect to all work perfectly in a new version of OSX. I know it takes time to work out all the little things, based on real world reports from users etc.
But Mail? Only Safari could be a less obscure feature.
What would interest me would be reports from Apple insiders that give us a window in to how such decisions are made within Apple.
As example, I recall one thread on the forum where an Apple developer who was working on Mavericks was apologizing for Mavericks bugs.
I'm sure there must be many factors at play in the decision of when to release a new OSX, and I would find it quite educational to better understand the calculations involved. Perhaps Apple users would be more understanding if we could see the larger picture that those inside Apple have to work with.