This plugin enhances the security and stability of your WordPress installation by delaying and controlling wp_mail() email submissions through a managed queue.
If your site exhibits unusual behavior — such as a spam bot repeatedly submitting forms — you will be alerted immediately.
<ul>
<li>Intercepts wp_mail() and places outgoing messages in a queue</li>
<li>Configure how many emails are sent and at what interval</li>
<li>Log all queued email submissions</li>
<li>Receive alerts when the queue grows unexpectedly</li>
<li>Receive alerts when WordPress is unable to send emails</li>
</ul>

Mail Queue

Take control of emails sent by WordPress. Queue outgoing emails and get notified instantly if your website is trying to send too many emails at once!

alxuta

By using this plugin you’ll be able to allow only a specific number of visitors to navigate through your website. 
The extra influx of visitors will be kept in a virtual queue until a new space will be available.
You could always use the following shortcode [virtual-queue-position], in order to display the current user position into the queue.
Installation
<ol>
<li>Upload the <code>virtual-queue</code> directory to the <code>/wp-content/plugins/</code> directory</li>
<li>Activate the plugin through the ‘Plugins’ menu in WordPress</li>
<li>Update the plugin settings</li>
<li>Set a Cron Job that will access the link https://domain.com/virtual-queue/maintenance/ where domain.com is your own domain name.</li>
<li>Go to Settings -> Permalinks / Permalink Settings and click Save Changes</li>
</ol>

Virtual Queue

By using this plugin you'll be able to allow only a specific number of visitors to navigate through your website.

Mail Queue vs Virtual Queue