If new comment posted in custom post – send notification to custom email from custom field

You can try something like this in your functions.php

function send_comment_email_notification( $comment_ID, $commentdata ) {
    $comment = get_comment( $comment_id );
    $postid = $comment->comment_post_ID;
    $master_email = get_post_meta( $postid, 'master_email', true);
    if( isset( $master_email ) && is_email( $master_email ) ) {
        $message="New comment on <a href="" . get_permalink( $postid ) . '">' .  get_the_title( $postid ) . '</a>';
        add_filter( 'wp_mail_content_type', create_function( '', 'return "text/html";' ) );
        wp_mail( $master_email, 'New Comment', $message );
    }
}
add_action( 'comment_post', 'send_comment_email_notification', 11, 2 );