PHPMailer

$mail->SMTPOptions = array(
        'ssl' => array(
            'verify_peer' => false,
            'verify_peer_name' => false,
            'allow_self_signed' => true
        )
    );

Phpmailer

<?php
// Import PHPMailer classes into the global namespace
// These must be at the top of your script, not inside a function
use PHPMailer\PHPMailer\PHPMailer;
use PHPMailer\PHPMailer\SMTP;
use PHPMailer\PHPMailer\Exception;

// Load Composer's autoloader
require 'vendor/autoload.php';

// Instantiation and passing `true` enables exceptions
$mail = new PHPMailer(true);

try {
    //Server settings
    $mail->SMTPDebug = SMTP::DEBUG_SERVER;                      // Enable verbose debug output
    $mail->isSMTP();                                            // Send using SMTP
    $mail->Host       = 'smtp1.example.com';                    // Set the SMTP server to send through
    $mail->SMTPAuth   = true;                                   // Enable SMTP authentication
    $mail->Username   = 'user@example.com';                     // SMTP username
    $mail->Password   = 'secret';                               // SMTP password
    $mail->SMTPSecure = PHPMailer::ENCRYPTION_STARTTLS;         // Enable TLS encryption; `PHPMailer::ENCRYPTION_SMTPS` encouraged
    $mail->Port       = 587;                                    // TCP port to connect to, use 465 for `PHPMailer::ENCRYPTION_SMTPS` above

    //Recipients
    $mail->setFrom('from@example.com', 'Mailer');
    $mail->addAddress('joe@example.net', 'Joe User');     // Add a recipient
    $mail->addAddress('ellen@example.com');               // Name is optional
    $mail->addReplyTo('info@example.com', 'Information');
    $mail->addCC('cc@example.com');
    $mail->addBCC('bcc@example.com');

    // Attachments
    $mail->addAttachment('/var/tmp/file.tar.gz');         // Add attachments
    $mail->addAttachment('/tmp/image.jpg', 'new.jpg');    // Optional name

    // Content
    $mail->isHTML(true);                                  // Set email format to HTML
    $mail->Subject = 'Here is the subject';
    $mail->Body    = 'This is the HTML message body <b>in bold!</b>';
    $mail->AltBody = 'This is the body in plain text for non-HTML mail clients';

    $mail->send();
    echo 'Message has been sent';
} catch (Exception $e) {
    echo "Message could not be sent. Mailer Error: {$mail->ErrorInfo}";
}

Wordpress debug config

// Enable WP_DEBUG mode
define( 'WP_DEBUG', true );

// Enable Debug logging to the /wp-content/debug.log file
define( 'WP_DEBUG_LOG', true );

// Disable display of errors and warnings
define( 'WP_DEBUG_DISPLAY', false );
@ini_set( 'display_errors', 0 );

// Use dev versions of core JS and CSS files (only needed if you are modifying these core files)
define( 'SCRIPT_DEBUG', true );

Wordpress nginx config

# Upstream to abstract backend connection(s) for php
upstream php {
        server unix:/tmp/php-cgi.socket;
        server 127.0.0.1:9000;
}

server {
        ## Your website name goes here.
        server_name domain.tld;
        ## Your only path reference.
        root /var/www/wordpress;
        ## This should be in your http block and if it is, it's not needed here.
        index index.php;

        location = /favicon.ico {
                log_not_found off;
                access_log off;
        }

        location = /robots.txt {
                allow all;
                log_not_found off;
                access_log off;
        }

        location / {
                # This is cool because no php is touched for static content.
                # include the "?$args" part so non-default permalinks doesn't break when using query string
                try_files $uri $uri/ /index.php?$args;
        }

        location ~ \.php$ {
                #NOTE: You should have "cgi.fix_pathinfo = 0;" in php.ini
                include fastcgi_params;
                fastcgi_intercept_errors on;
                fastcgi_pass php;
                #The following parameter can be also included in fastcgi_params file
                fastcgi_param  SCRIPT_FILENAME $document_root$fastcgi_script_name;
        }

        location ~* \.(js|css|png|jpg|jpeg|gif|ico)$ {
                expires max;
                log_not_found off;
        }
}

Wordpress PHPMailer config

add_action( 'phpmailer_init', 'setup_phpmailer_init' );
function setup_phpmailer_init( $phpmailer ) {
    $phpmailer->Host = 'HOSTNAME'; // for example, smtp.mailtrap.io
    $phpmailer->Port = 587; // set the appropriate port: 465, 2525, etc.
    $phpmailer->Username = 'YOURUSERNAME'; // your SMTP username
    $phpmailer->Password = 'YOURPASSWORD'; // your SMTP password
    $phpmailer->SMTPAuth = true;
    $phpmailer->SMTPSecure = 'tls'; // preferable but optional
    $phpmailer->IsSMTP();