Ok...
With valuable help from TheOracle (thank you!), this is working very well for extracting the emails from ecards. Here's the code I'm using:
<?php // PLUGIN_TITLE: Ecard Emails
$nozip = 1;
define('IN_CP', 1);
define('ROOT_PATH', "./../../");
define('ROOT_PATH_ADMIN', "./../");
require(ROOT_PATH_ADMIN.'admin_global.php');
show_admin_header();
$anzahl = 0;
$sql = "SELECT postcard_recipient_email, postcard_sender_email
FROM ".POSTCARDS_TABLE."";
$result = $site_db->query($sql);
$anz = $site_db->get_numrows($result);
if ($anz) {
$fp = @fopen('addresses/emails.csv', 'w+');
while ($sql_array = $site_db->fetch_array($result, MYSQL_NUM)) {
$text = implode("," , $sql_array) . ",";
$list = str_replace("," , "\n" , $text);
@fwrite ($fp, $list);
} ###### End of while statement.
@fclose($fp);
echo "<a href=\"".$site_sess->url(ROOT_PATH_ADMIN."plugins/addresses/emails.csv")."\">Download</a>";
} ###### End of if statement.
show_admin_footer();
?>