Visitors Contacting Authors

Ive done this before using both contact form 7 and Grunion Contact Form plugins, when using contact form 7 i created a new tag to send the email to the post author:

//contact form 7 author email
wpcf7_add_shortcode('author_email', 'wpcf7_expert_email_shortcode_handler', true);

function wpcf7_expert_email_shortcode_handler($tag) {
    if (!is_array($tag)) return '';

    $name = $tag['name'];
    if (empty($name)) return '';
    global $post;
    //get author email:
    $author_mail = get_the_author_meta('user_email',$post->post_author);
    $html="<input type="hidden" name="" . $name . '" value="' . $author_mail . '" />';
    return $html;
}

/*
* Usage: in contact form 7 form area add
* [author_email unique_name]
* and in to email address add
* [unique_name]
*/

and with Grunion i called it directly from the template using:

global $post;
$author_mail = get_the_author_meta('user_email',$post->post_author);
echo do_shortcode('[contact-form subject="message from.'$post->post_title.'" to="'.$author_mail.'"]');