I first noticed this bug in Assault, but it works in all game modes. Mail of Renewal will activate on tower kills (or tower kill assists, i.e. i helped kill the tower), giving you the full health and mana. This is clearly a bug, since the item's description said it only activated on kills and assists. As far as balance is concerned, I think this behavior should stay in the game, and the item's description should be changed to match it, but that's just my personal opinion.