@deyashalash: I expect you’re referring to this infamous ball-ache here…
When it comes to my email history though, I prefer to have them and not need them rather than need them and not have them (unfortunately I learnt that the hard way), so aside from a couple of spam emails per year that slip through the net I delete absolutely nothing!
The other thing that comes to mind is:
Settings → Mail → “MESSAGES: Ask Before Delete” (6th one or so down).
I thought that was disabled by default, but I’m too worried about shafting important emails by accident (see above for why heh).
Oh, actually, there is one exception I make... And it’s when moronic companies send my password back to me in plain text… I may even go search my inbox right now for a couple of my regular passwords just in case I ever missed any of those…