In your original post you said you're filtering "To"; now you say "To:" is hardcoded.

For preventing mass mailing you may generate pictures on the fly and ask the "USER" to complete a field of the form with the content of the picture (text or number)

There's no point in using SSL unless you want to protect the contents of the message or you are using some means of auth (user/pass secret key etc) which I don't think is the case.