thebluecube

This plugin is intended to be useful more for developers than other types of users.
Have you ever had a client who likes to be able to independently update the content of the emails that your code is sending out programmatically? If so, this plugin might help you make your client happy.
How to use:
When you activate the plugin, it adds a custom post type to your website called “Email Templates”. You can create your templates using this custom post type. Templates can contain placeholders for your code to fill them up with desired values. A placeholder sample would be {USERNAME} or {USER_EMAIL}. Please note, placeholders should not contain lowercase letters.
This plugin creates a global instance of the BlueCube\Email_Template class which is called $bluecube_email_template. Once you have created a template, you can use the sendEmail() method of the instance to send out your emails.
<pre><code>$email_args = array(
 'template_title' => 'your-template-title',
 'to' => $email,
 'variables' => array('username' => 'JohnDoe', 'user_email' => 'test@test.com'),
);
$bluecube_email_template->sendEmail($email_args);
</code></pre>

Blue Cube Email Templates

This plugin provides an email template system for developers to send programmatic emails which their clients can easily update the emails content.

Brian Mutende

Email Customizer is a lightweight Email Customizer Plugin For WordPress. Now works with WooCommerce 
★★★★★
With this plugin, you can convert the plain text emails that WordPress sends into beautiful HTML emails.
These include the emails that WordPress sends to users after they create an account on your website or request to reset their password.
The email customizer plugin wraps all your WordPress emails inside of a HTML template. As a result, the colors and appearance of your emails will match that of your website.
You can edit the email template and set text sizes, colors and backgrounds. You can also define custom CSS for your emails and it will be inlined inside the emails.
Noptin is a free and lightweight WordPress newsletter plugin. <a href="https://wordpress.org/plugins/newsletter-optin-box/" rel="ugc">Learn more</a>.

Email Customizer

Easily replace the plain text WordPress emails with beautiful HTML emails that match your brand colors. All without writing a single line of code.

Blue Cube Email Templates vs Email Customizer