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 and can optionally pause delivery automatically while you investigate.
<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>Pause the queue without disabling the plugin completely</li>
<li>Log queued emails, sent emails, alerts, and queue events</li>
<li>Receive alerts when the queue grows unexpectedly</li>
<li>Optionally auto-pause the queue when an alert is triggered</li>
<li>Send selected queued emails immediately from the backend</li>
<li>Automatically prioritize WordPress password reset emails</li>
<li>Receive alerts when WordPress is unable to send emails</li>
</ul>

Mail Queue

Gain full control over your WordPress emails. Queue outgoing emails and get alerts when your site tries to send unusually high volumes.

Milan Dinić

<a href="https://milandinic.com/wordpress/plugins/simple-email-queue/" rel="nofollow ugc">Plugin homepage</a> | <a href="https://milandinic.com/" rel="nofollow ugc">Plugin author</a> | <a href="https://shop.milandinic.com/downloads/simple-email-queue-plus/" rel="nofollow ugc">Plus Version</a>
Simple Email Queue is a WordPress plugin that is used to pass restrictions set by your host on number of sent emails in given period.
This basic free version is only useful for developers that can extend it and use it in their code. If you want to use it in full capacity, consider getting <a href="https://shop.milandinic.com/downloads/simple-email-queue-plus/" rel="nofollow ugc">plus version</a>.
<h3>Usage</h3>
First, you need to set what is maximum number of emails that can be sent in a given period. By default, it sends 10 emails in 6 minutes. This means that if you want to send 15 emails in 6 minutes, only 10 emails are sent while rest 5 emails are put in queue and sent in the next period, after 6th minute has passed.
You can change this limits by using filters from you code. Filter <code>simple_email_queue_max</code> is used to set maximum number of emails that are sent in period. It expects positive integer to be passed.
To change period length, you can use filter <code>simple_email_queue_interval</code>. It also expects positive integer to be passed but please take care that this number is of seconds, not minutes (for example, if your period is 30 minutes, you would pass <code>1800</code>).
Different hosts use different limits. Please consult your host’s documentation or support to find this out.
If you want user interface in your admin area, use <a href="https://shop.milandinic.com/downloads/simple-email-queue-plus/" rel="nofollow ugc">plus version</a>.
To use this limits, you need to use function <code>simple_email_queue_add()</code> instead of built-in function <code>wp_mail()</code>. Both accept same parameters. 
Emails that are sent using <code>wp_mail()</code> function are not sent using queue and are not counted for limits. If you want that all emails are sent using queue, even those sent using <code>wp_mail()</code> function, use <a href="https://shop.milandinic.com/downloads/simple-email-queue-plus/" rel="nofollow ugc">plus version</a>.
And it’s on <a href="https://github.com/dimadin/simple-email-queue" rel="nofollow ugc">GitHub</a>.

Simple Email Queue

Put email in queue and send it one by one, by limits.

Mail Queue vs Simple Email Queue