The Mail notification will show at the top of the Minimap in the Bottom-Middle. It is very small and easy to oversee.
Some tips to your interface:
You really should change some things there. It is really to blink for me.
For Omen and Recount it gives another Addon named Skada, that is out of the box compatible with nUI.
There are many other things, but I don't play a hunter so I don't can say what you need. I only say it, because later in game, if you go into raid instances with 25 people, you need a really got look to what happen.
To better recognize some things, you should try TiTanPanel. There is a Mod, showing you if you get mail and many other things. Sorry I haven't any link yet. Maybe someone can give it to you.
Hopefully that nUI has won a new player who loves Scott's Addon.
