I am currently writing a music blog. The administrator posts a new article every 2-3 days. Once the administrator posts an article, a mass email will be sent to around 5000 subscribers immediately.

What is the best way to implement the mass mail feature?

Does the following function work?

function massmail() 
{
  $content = '...';
  foreach ($recipients as $r) {
    $_content = $content . '<img src="http://xxx/trackOpenRate.php?id='.$r.'">';
    mail($r, 'subject', $_content);
  }
}

Another question: If all 5000 subscribers are using Yahoo Mail, will Yahoo treat it as a DDOS attack and block the IP address of my SMTP server?