Google is not being very helpful here. I'd like to compose emails with :set spell on, but for all other files, :set spell should be off. I am using mutt, and Vim as the editor for composing emails.
In fact, I'm curious, how does Vim know that it's an email I'm composing? Is there a command-line parameter of the type of file being edited? How does mutt know what to pass in?
You can use an autocmd in your ~/.vimrc that gets triggered by the detected file type:
autocmd FileType mail set spell