Send action stuck in the WAITING state

The plugin is using the SMTP method that points to my SMTP server. Test emails from the Outgoing Email Settings page and the newsletter with the custom template work fine. However, when I use the "Send" button on the newsletters are, the emails remain in a waiting state. Wordpress is running on IIS 7. Since I'm on a Windows server, I haven't scheduled CRON jobs and CRON is disabled in the plugin.