More and more people are emailing from their phone/pda/small device of some kind.
As such, they often have something on the bottom such as:
Sent from my phone; pardon brevity.
This gives you licence to write a very short reply, and not care about grammar as much ;)
So, why not fake it? If you don’t want to say much, add that to the bottom of your email, and Bob’s your uncle! :)