A simple class to override WP's email functionality to debug/inspect what will be sent. Put in your mu-plugins directory.

sd_mail_killer.php

<?php

/*
Plugin Name: Mail Killer/Logger
Description: A simple class to override WP's email functionality to debug/inspect what will be sent. Put in your mu-plugins directory.
Version: 1.0
Author: Matt Wiebe
Author URI: http://somadesign.ca/
License: A "Slug" license name e.g. GPL2
*/

class SD_Mail_Killer {
	
	private $phpmailer;
	private $kill_email = true; // true: email will not be sent
	private $log = true; // true: the PHPMailer Object will be written to your error_log for debugging purposes.
	
	function __construct() {
		add_action('phpmailer_init', array($this, 'replace'), 10, 1);
	}
	
	function Send() {
		if ( ! $this->kill_email )
			$this->phpmailer->Send();
		if ( $this->log )
			error_log(print_r($this->phpmailer, true));
		do_action('sd_mail_killer', $this->phpmailer);
	}
	
	function replace($phpmailer) {		
		$this->phpmailer = $phpmailer;
		$phpmailer = $this;
	}
}
new SD_Mail_Killer;