I'm definitely guilty of using shorthand (e.g. y for yes, u for you etc.) but I really only ever use it when instant messaging. I also *know* everybody who would get an IM from me, so they know what kind of person I am and I really don't need to prove that I can spell.
I also have worked in client support and therefore have run into a great deal of people that pissed me off for various reasons....but I don't ever see the reason to tell my clients that they are stupid/dumb or to fuck off. Maybe you have too many clients
Is this person not a native English speaker? That might explain the bad grammar...