author_email
author_email is a filter on the output of comment_author_email(). Note that you can use the get_comment_author_email hook instead and you can cover both comment_author_email() and get_comment_author_email().
Context:
File: wp-includes/comment-functions.php
function get_comment_author_email() {
global $comment;
return apply_filters('get_comment_author_email', $comment->comment_author_email);
}
function comment_author_email() {
echo apply_filters('author_email', get_comment_author_email() );
}
This hook is a filter which means that information is passed through it, and then used by WordPress. Your function needs to accept that information, and return it. Using add_filter('author_email', 'your_function'); helps you to remember this distinction. When you are passing an ID, it is assumed that you will return the ID as it was given to you. With filters that pass strings or arrays, you may manipulate the information before passing it along.