Some of my user requested to post via email, but I didn't want to create big hole into my blog, so I improved wp-mail.php. Now he can publish his messages, and I'am happy again, because unauthorized users can't post to my blog.
* each post is authentified by email of registered author and password
* author's password must be present in subject after --- separator (spaces are trimmed)
Subject: password protected posts via email --- my_password