A simple mu–plugin snippet for overriding and logging WP emails in development.

wp-email-log.php

<?php

class R_Mailer {
	function Send() {
		error_log( wp_debug_backtrace_summary() );
	}
}

add_action( 'phpmailer_init', function( &$mailer ) {
	$mailer = new R_Mailer();
} );