Is it okay to be mad at my friend who constantly asks for my help writing emails?
Why are you mad at him?
I can't take this man anymore, he is an unwanted headache.
I know you're upset, but it's expected of friends to rely on one another. You should try being more friendly to him.
Fuck that. I'm not going to be nice to him.
Is it worth losing them as a friend because of your frustration? Teach them how to properly compose their own emails and answer any questions they may have. Tell them you'll help them but can't be doing their work for them any longer.