WordPress does not provide any function that would check the commenter’s eMail against e.g. the blog owner’s eMail. That way, anybody could use the admin’s name and eMail to leave a comment in the name of the blog owner. Since these blog owner’s comments are often highlighted, this impersonation is quite crucial.

Here is a way to disallow using a distinct eMail without being logged in:

1. Open wp-comments-post.php from the WP root directory.
2. Search for the following code (line 53 in WP 2.6.2):

   if ( get_option('comment_registration') )
   wp_die( __('Sorry, you must be logged in to post a comment.') );

3. After those lines, insert this and put in the eMail that should be checked (repeat these lines if you want several eMails to be checked):

   if ( strtolower($comment_author_email) == strtolower('email@to-be-check.ed') )
   wp_die( __('Sorry, but you must be logged in to use that email. Nice try, though...  ') );

Note:
If you are using Filosofo Comments Preview ^[2], you have to open that plugin file and do the same changes. The same would be necessary for any plugin that bypasses wp-comments-post.php.

Another note:
I wonder if I could use the “preprocess_comment” filter to add this functionality, so that the core WordPress files do not have to be touched.

Happy note:
Eureka! Bingo! Works! My first plugin!

Inspired by the article Prevent author impersonation in WordPress comments ^[3].