要在Ubuntu中安装PEAR的Mail.php，运行以下命令:

    sudo apt-get install php-pear
    sudo pear install mail
    sudo pear install Net_SMTP
    sudo pear install Auth_SASL
    sudo pear install mail_mime

我知道这是一个老问题，但它仍然有效，我看到的所有答案都显示了基本的身份验证，这是不推荐的。下面是一个示例，展示如何通过谷歌的Gmail服务器使用PHPMailer和XOAUTH2身份验证，使用SMTP发送电子邮件:

//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 = 'someone@gmail.com';
$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('someone@gmail.com', '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!';
}

参考:PHPMailer示例文件夹

我尝试了@shasi kanth提供的建议，但没有奏效。我读了文档，做了一些改动。所以我设法通过Gmail使用这段代码发送邮件，其中vendor/autoload.php是由作曲家与作曲家要求“swiftmailer/swiftmailer:^6.0”:

<?php
     require_once 'vendor/autoload.php';
     $transport = (new Swift_SmtpTransport('smtp.gmail.com', 465, 'ssl'))->setUsername ('SendingMail')->setPassword ('Password');

     $mailer = new Swift_Mailer($transport);

     $message = (new Swift_Message('test'))
      ->setFrom(['Sending mail'])
      ->setTo(['Recipient mail'])
      ->setBody('Message')
  ;

     $result = $mailer->send($message);
    ?>

I have a solution for GSuite accounts that doesnt have the "@gmail.com" sufix. Also I think it will work for GSuite accounts with the @gmail.com but havent tried it. First you should have the privileges to change the option "allos¿w less secure app" for your GSuite account. If you have the privileges (you can check in account settings->security) then you have to deactivate "two step factor authentication" go to the end of the page and set to "yes" for allow less secure applications. That's all. If you dont have privileges to change those options the solution for this thread will not work. Check https://support.google.com/a/answer/6260879?hl=en to make changes to "allow less..." option.

SwiftMailer可以使用外部服务器发送电子邮件。

下面是一个演示如何使用Gmail服务器的例子:

require_once "lib/Swift.php";
require_once "lib/Swift/Connection/SMTP.php";

//Connect to localhost on port 25
$swift =& new Swift(new Swift_Connection_SMTP("localhost"));


//Connect to an IP address on a non-standard port
$swift =& new Swift(new Swift_Connection_SMTP("217.147.94.117", 419));


//Connect to Gmail (PHP5)
$swift = new Swift(new Swift_Connection_SMTP(
    "smtp.gmail.com", Swift_Connection_SMTP::PORT_SECURE, Swift_Connection_SMTP::ENC_TLS));

使用Swift mailer，很容易通过Gmail凭据发送邮件:

<?php
require_once 'swift/lib/swift_required.php';

$transport = Swift_SmtpTransport::newInstance('smtp.gmail.com', 465, "ssl")
  ->setUsername('GMAIL_USERNAME')
  ->setPassword('GMAIL_PASSWORD');

$mailer = Swift_Mailer::newInstance($transport);

$message = Swift_Message::newInstance('Test Subject')
  ->setFrom(array('abc@example.com' => 'ABC'))
  ->setTo(array('xyz@test.com'))
  ->setBody('This is a test mail.');

$result = $mailer->send($message);
?>