TAKAHIRO

EmailZap is a simple and lightweight plugin that replaces WordPress’s default mail sending with SMTP. 
You can easily configure SMTP host, port, encryption method, authentication, username, and password from the admin settings page.
Passwords are encrypted using AES-256-CBC before being saved to the database, enhancing your site’s security.

EmailZap

A lightweight and secure plugin that switches your WordPress email sending to SMTP. Configure your SMTP server easily from the admin panel.

holzhannes

pCoLaSD

This plugin configures WordPress and WordPress MU to use a SMTP server when sending emails instead of the default PHP <code>mail()</code> function.
You will configure your SMTP settings in your <code>wp-config.php</code> file instead of the settings page. 
The advantage is that no admin of your blog can read the settings. 
And you only have to place your settings once in cases of a WordPress MU installation.
A sample configuration:
<pre><code>/**
 * WordPress SMTP server
 */
define('WP_SMTP_HOST', 'mail.example.com');
define('WP_SMTP_PORT', 25); // obligatory - default: 25
define('WP_SMTP_ENCRYPTION', 'tls'); // obligatory ('tls' or 'ssl') - default: no encryption
define('WP_SMTP_USER', 'username'); // obligatory - default: no user
define('WP_SMTP_PASSWORD', 'password'); // obligatory - default: no password
define('WP_SMTP_FROM', 'John Doe <john.doe@example.com>'); // obligatory - default: no custom from address
define('WP_SMTP_REPLYTO', 'Jane Doe <jane.doe@example.com>'); // obligatory - default: no custom reply to address
</code></pre>

WP SMTP Config

Configure an external SMTP server in your config file.

EmailZap vs WP SMTP Config