WP: Log outgoing emails.php

<?php
/**
 * Logs error to file
 * 
 * @uses `error_log` function to log on file
 * 
 * @param string $error message of the error
 * @param string $type type of the error message
 */
function log_error( $error, $type = 'error' ) {
    $path = dirname( __FILE__ ) . '/mail_log.txt';
    $msg = sprintf( "[%s][%s] %s\n", date( 'd.m.Y h:i:s' ), $type, $error );
    error_log( $msg, 3, $path );
}

/**
 * Log the mail to text file
 * 
 * @uses `wp_mail` filter
 * @param array $mail
 */
function wedevs_mail_log( $mail ) {

    $message = "to: {$mail['to']} \nsub: {$mail['subject']}, \nmsg:{$mail['message']}";
    log_error( 'mail', $message );

    return $mail;
}

add_filter( 'wp_mail', 'wedevs_mail_log', 10 );

// special thnx to http://tareq.wedevs.com/2012/03/log-wordpress-outgoing-mails-for-debugging/