SMTP.com

<a href="https://smtp.com" rel="nofollow ugc">SMTP.com</a> was the first company to provide email delivery at scale for customers unsatisfied with traditional sending methods. With over 22 years of email delivery success, SMTP.com is the trusted transactional and email relay provider for over 100,000 businesses worldwide. This WordPress integration for SMTP.com enables you to send and track your emails seamlessly.
Don’t worry about maintaining your email server or taking care of its IP reputation, forget about dealing with ISPs like Gmail, Yahoo or Microsoft in attempting to understand why your email isn’t reaching its destination. SMTP.com manages all this on your behalf. Simply create a SMTP.com account to <a href="https://registration.smtp.com/?afid=wpplugin" rel="nofollow ugc">get started</a> and set up this plugin. Although this plugin is free you will need a paid subscription with SMTP.com to send emails from your WordPress site.
You can send emails from your WordPress site using the SMTP.com API or the standard SMTP protocol. Our plugin will suggest the best way for you to send email even if your provider has restricted access to outbound internet ports. Either way, SMTP.com will still ensure your email gets delivered.
Another useful feature provided with our WordPress plugin is the ability to track your email delivery from within the plugin itself. See the results of your recent sends to ensure successful delivery or identify the reason why your particular email hasn’t been delivered.


Didn't work with the API key, but managed with the login information.


Good

SMTP.com is a powerful and reliable SMTP delivery service that enables you to send and track high volume emails effortlessly.

Birmania

hildende

This plugin adds emails to a mailing queue instead of sending immediately. This speeds up sending forms for the website visitor and lowers the server load. 
Emails are stored as files which are deleted after emails are sent.
You can send all outgoing emails via an SMTP server or (the WordPress standard) PHP function <a href="http://php.net/manual/en/function.mail.php" rel="nofollow ugc">mail</a>, and either use <a href="https://codex.wordpress.org/Function_Reference/wp_cron" rel="nofollow ugc">wp_cron</a> or a cronjob (if your server/hoster supports this) to process the queue.
Plugin requires PHP 5.4 or above.
Tools:
<ul>
<li>You can send test mails to test your setup.</li>
<li>You can process the mailing queue manually instead of waiting for cronjob.</li>
</ul>
Supervisors: 
* You can display basic informations about mail processing performances 
* You can display the mailing queue in the backend to see emails that will be sent with next processing. 
* You can display the invalid mails and delete or retry them. 
* You can display the sent mails and delete them.
Coming soon:
<ul>
<li>Storing mailing data in database instead of files.</li>
<li>Using plugin for SMTP mails without using mailing queue.</li>
</ul>
Feel free to suggest features or send feedback in the <a href="https://wordpress.org/support/plugin/smtp-mailing-queue" rel="ugc">support section</a>, via <a href="mailto:antoine.brultet@gmail.com" rel="nofollow ugc">email</a> or by creating a pull request on <a href="https://github.com/Birmania/smtp-mailing-queue" rel="nofollow ugc">github</a>.

SMTP Mailing Queue

Add emails to a mailing queue instead of sending immediately to speed up sending forms for the website visitor and lower server load.

SMTP.com vs SMTP Mailing Queue