acsnaterse

Disables all email through wp_mail() on current installation. Email will by disabled by default, but can be overridden in the settings.
<h3>Features</h3>
<ul>
<li>Disables all email by default</li>
<li>Enable email again by matching the given hostname with the current server hostname. </li>
<li>Override the <code>$to</code> address (or multiple, comma sperated addresses), so you can use that for testing purposes.</li>
</ul>
Note: The plugin performs a check on the <code>$_SERVER['HTTP_HOST']</code> variable.

WordPress Disable Email plugin

Disables all email through wp_mail() on current installation. Email will by disabled by default, but can be overridden in the settings.

rajuru

SparkPost

The <a href="https://www.sparkpost.com/" rel="nofollow ugc">SparkPost</a> email delivery service offers best in class deliverability to ensure your mail hits the inbox, live analytics to review, track and optimize your email activities, as well as highest performance when you need it most: always.
When the SparkPost plugin is enabled, all outgoing email from your WordPress installation is sent through your SparkPost service. From within <a href="https://app.sparkpost.com/" rel="nofollow ugc">the SparkPost UI</a> or <a href="https://app.eu.sparkpost.com/" rel="nofollow ugc">SparkPost EU UI</a>, you can then watch your campaigns unfold live by tracking your engagement metrics, learn what your audience responds to and even integrate more deeply with your app using the SparkPost API.
Requirements 
– PHP 7.0 or later 
– WordPress 5.5 or later

Send all your email from WordPress through SparkPost, the most advanced email delivery service.

WordPress Disable Email plugin vs SparkPost