WordPress Mail Queue was created to resolve an issue created by the age old concerns about spam emails.
Subscribe2 HTML is a comprehensive subscription management system for WordPress and ClassicPress blogs that sends emails, often lots of emails. All hosting providers have some restrictions on emails sent from their servers. They do this to maintain the server uptime and also maintain a good level of credibility. If you are sending lots of emails you may breach these restrictions and your emails fail to send.
WordPress Mail Queue allows your emails to be stored on the server in a database and sent in stages. You can configure how may emails are sent and how often this happens to ensure you remain within your hosing providers limits and improve your email deliverability.