cubefeed

The problem: SMTP email delivery by some hosting providers can be extremely slow. This plugin reconfigures the wp_mail() function to send email using HTTPS (via Elasticemail API) instead of SMTP and creates an options page that allows you to specify various options.
Plugin Settings Options:
<ul>
<li>Enable or Disable Plugin.</li>
<li>Elasticemail API username</li>
<li>Elasticemail assigned API Key</li>
<li>Elasticemail REST API URL</li>
<li>Your account channel variable(optional)</li>
</ul>

ElasticEmail Plugin

Reconfigures the wp_mail() function to send email using HTTPS (via Elasticemail) instead of SMTP and creates an options page to manage the settings.

socketlabs

SocketLabs is a cloud-based email delivery platform built to help you send your marketing and transactional email from one platform. SocketLabs provides optimized email delivery at scale, real-time tracking and analytics, reliable Email APIs for developers, and a suite of Email Marketing Tools that will make sending your next email campaign a breeze.
The SocketLabs WordPress Plugin allows you to easily route email generated by WordPress through SocketLabs rather than the default wp_mail() function that WordPress provides.
Use the SocketLabs plugin if:
<ol>
<li>Your mail from WordPress is failing to be delivered and you need an easy way to get your email to the inbox.</li>
<li>You’re looking for better Inbox placement over an optimized email network.</li>
<li>You would like to get more insight into the performance of your email from our detailed reports in the SocketLabs Control Panel.</li>
<li>Your WordPress instance has been blacklisted and you need to get your mail flowing again.</li>
<li>Your WordPress site is exceeding the number of emails allowed by your ISP.</li>
</ol>
By using the SocketLabs plugin, you can finally take advantage of improved deliverability from WordPress, while benefiting from our powerful delivery platform that’s built to get your emails to the inbox. And in your SocketLabs Control Panel, you’ll be able to take advantage of our expanded feature set, including in-depth tracking and analytics, a Drag and Drop Email Designer, list management, and so much more!
For more details, please visit our official documentation here: 
https://www.socketlabs.com/blog/configure-wordpress-plugin/
<h3>How do I create a SocketLabs account?</h3>
You can sign up for a free account here https://socketlabs.com/signup_form/?plan=Free.
<h3>Help & Support</h3>
Visit our Support System if you have any questions, the SocketLabs Support Team is happy to help — https://support.socketlabs.com/

SocketLabs

The SocketLabs WordPress Plugin allows you to easily send email generated by WordPress through the SocketLabs Email Delivery Service.

ElasticEmail Plugin vs SocketLabs