PHP在线发送邮件的难点：为什么mail()函数不能满足需求？

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       = 'smtp.example.com';                     //Set the SMTP server to send through
    $mail->SMTPAuth   = true;                                   //Enable SMTP authentication
    $mail->Username   = 'username@example.com';                  //SMTP username
    $mail->Password   = 'secret';                               //SMTP password
    $mail->SMTPSecure = PHPMailer::ENCRYPTION_STARTTLS;         //Enable TLS encryption; `PHPMailer::ENCRYPTION_SMTPS` also accepted
    $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

    //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}";
}
?>