In spite of going through the aspell documentation, I cannot get it to skip over quoted text, as the email mode is expected to do. I have the following in my .emacs.d file:
(setq ispell-program-name "aspell")
(add-hook 'mail-mode-hook (lambda ()
(setq ispell-extra-args '("--mode=email"))))
Aspell, however, keeps spell checking the quoted message, even although C-h v ispell-extra-args returns Its value is ("--mode=email"). I should also point out that executing aspell from the command line does not ignore the --mode=email option, so there seems not to be anything wrong with aspell itself.
I think, that you need to look onto ispell-message function from the same package. It will spell check body of your message & Subject header, but will exclude other headers & cited mail. In function's description (C-h f ispell-message) you can find examples of hooks for mail-send & message-send commands...