Arnt is short (slang) for Are Not. It is used mostly in personal emails that are non-job related. In a professional environment it is rarely used accept in a joking manner...However it is quite common outside of the corporate world. It can also be used to discribe someone.
Arnt you just the most worthless employee to ever set foot in our corporation.
We arnt going to be happy until this word is globally accepted.
We arnt including you in this game of basketball because you suck.
We arnt going to remove your account from the server until you admit your life is worthless.