<?php
function send($message, $to) {
    require_once "Mail.php";
    $from = "Info <info@example.com>";
    $subject = "Newsletter";
    $host = "****";
    $username = "****";
    $password = "****";
    $headers = array (
        'From' => $from,
        'To' => $to,
        'Subject' => $subject,
        'Content-Type' => 'text/html; charset="utf-8"'
    );
    $smtp = Mail::factory('smtp',
        array (
            'host' => $host,
            'auth' => true,
            'username' => $username,
            'password' => $password
        )
    );
    $smtp->send($to, $headers, $message);
}


foreach (DB:getEmails() as $email) {
    $messageData->user = $email;
    // In `letter.php` instead of using `^user^`, use `<?= $messageData->user ?>`.  You'll have more    flexibility
    $message = $foo = include 'messages/letter.php';
    send(str_replace('^user^',$email,$message),$email);
}
?>