Currently, to post by email, you send an email with the post in the body and the subject is the entry' title. The problem with post-by-email is that every email sent to the specified address is posted, even if it were spam. Why not set an email posting password. The first line of the email body would be your password, and it would only be posted if that password were the first line, otherwise it would be either ignored or entered in the "draft" state.
So, the email could be
Subject: Test post
Body:
mypassword123
the body of the blog posting....