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.

This plugin when activated automatically redirects all email sent through wp_mail/phpmailer to the user currently logged in or the site administrator.
The redirected email contains information about where the original email would have been sent to.
Use this plugin if you want to test email-sending features without actually sending emails our to anyone but yourself. I wrote this plugin to ease debugging with subscribe2.
Since Version 1.2 you can choose whether to redirect all email to the currently logged in user (default), the site administrator (only wpmu) or the blog administrator.

Mail Debug

Redirects all email sent through wordpress to the user currently logged in or the site administrator.

SMTP.com vs Mail Debug