I know this is an old question but it's still active and all the answers I saw showed basic authentication, which is deprecated. Here is an example showing how to send via Google's Gmail servers using PHPMailer with XOAUTH2 authentication:
//Import PHPMailer classes into the global namespace
use PHPMailer\PHPMailer\PHPMailer;
use PHPMailer\PHPMailer\SMTP;
use PHPMailer\PHPMailer\OAuth;
//Alias the League Google OAuth2 provider class
use League\OAuth2\Client\Provider\Google;
//SMTP needs accurate times, and the PHP time zone MUST be set
//This should be done in your php.ini, but this is how to do it if you don't have access to that
date_default_timezone_set('Etc/UTC');
//Load dependencies from composer
//If this causes an error, run 'composer install'
require '../vendor/autoload.php';
//Create a new PHPMailer instance
$mail = new PHPMailer();
//Tell PHPMailer to use SMTP
$mail->isSMTP();
//Enable SMTP debugging
//SMTP::DEBUG_OFF = off (for production use)
//SMTP::DEBUG_CLIENT = client messages
//SMTP::DEBUG_SERVER = client and server messages
$mail->SMTPDebug = SMTP::DEBUG_SERVER;
//Set the hostname of the mail server
$mail->Host = 'smtp.gmail.com';
//Set the SMTP port number:
// - 465 for SMTP with implicit TLS, a.k.a. RFC8314 SMTPS or
// - 587 for SMTP+STARTTLS
$mail->Port = 465;
//Set the encryption mechanism to use:
// - SMTPS (implicit TLS on port 465) or
// - STARTTLS (explicit TLS on port 587)
$mail->SMTPSecure = PHPMailer::ENCRYPTION_SMTPS;
//Whether to use SMTP authentication
$mail->SMTPAuth = true;
//Set AuthType to use XOAUTH2
$mail->AuthType = 'XOAUTH2';
//Fill in authentication details here
//Either the gmail account owner, or the user that gave consent
$email = '[email protected]';
$clientId = 'RANDOMCHARS-----duv1n2.apps.googleusercontent.com';
$clientSecret = 'RANDOMCHARS-----lGyjPcRtvP';
//Obtained by configuring and running get_oauth_token.php
//after setting up an app in Google Developer Console.
$refreshToken = 'RANDOMCHARS-----DWxgOvPT003r-yFUV49TQYag7_Aod7y0';
//Create a new OAuth2 provider instance
$provider = new Google(
[
'clientId' => $clientId,
'clientSecret' => $clientSecret,
]
);
//Pass the OAuth provider instance to PHPMailer
$mail->setOAuth(
new OAuth(
[
'provider' => $provider,
'clientId' => $clientId,
'clientSecret' => $clientSecret,
'refreshToken' => $refreshToken,
'userName' => $email,
]
)
);
//Set who the message is to be sent from
//For gmail, this generally needs to be the same as the user you logged in as
$mail->setFrom($email, 'First Last');
//Set who the message is to be sent to
$mail->addAddress('[email protected]', 'John Doe');
//Set the subject line
$mail->Subject = 'PHPMailer GMail XOAUTH2 SMTP test';
//Read an HTML message body from an external file, convert referenced images to embedded,
//convert HTML into a basic plain-text alternative body
$mail->CharSet = PHPMailer::CHARSET_UTF8;
$mail->msgHTML(file_get_contents('contentsutf8.html'), __DIR__);
//Replace the plain text body with one created manually
$mail->AltBody = 'This is a plain-text message body';
//Attach an image file
$mail->addAttachment('images/phpmailer_mini.png');
//send the message, check for errors
if (!$mail->send()) {
echo 'Mailer Error: ' . $mail->ErrorInfo;
} else {
echo 'Message sent!';
}
Reference: PHPMailer examples folder
'phpmailer_init'action hook. It's a convenient way to set up WordPress for SMTP mail or Amazon SES (which supports SMTP connections). – Gass