more efficient alternative to mail()

Question regarding php's mail function. I've created a targetted mail system for a client, that uses the mail() function to send the the customer list. I find it's ok for a few hundred names, however the client has a list of over 10000 and I know that would be way to taxing on the server, as some tests have shown. I got the following function off the php.net site and I'm wondering if something like this will be significantly more efficient than using the mail function. function sendmail($to='', $subject='', $message='', $headers='', $extra='') { $fd = popen("/usr/sbin/sendmail -t $extra", 'w'); fputs($fd, "To: $to\n"); fputs($fd, "Subject: $subject\n"); fputs($fd, "X-Mailer: PHP4\n"); if ($headers) { fputs($fd, "$headers\n"); } fputs($fd, "\n"); fputs($fd, $message); pclose($fd); } At this point using perl or another language is not an option. Any advice or links would be appreciated. Cheers, Jeff.