使用PHP邮件程序发送带有附件的邮件时出错:事务失败:缺少起始边界SMTP代码: 554

<?php
$to = 'recipient@example.com';
$subject = '邮件主题';
$message = '邮件内容';

// 生成起始边界
$boundary = md5(time());

// 设置邮件头部信息
$headers = "From: sender@example.com\r\n";
$headers .= "MIME-Version: 1.0\r\n";
$headers .= "Content-Type: multipart/mixed; boundary=\"$boundary\"\r\n";

// 构建邮件内容
$body = "--$boundary\r\n";
$body .= "Content-Type: text/plain; charset=UTF-8\r\n";
$body .= "Content-Transfer-Encoding: 7bit\r\n\r\n";
$body .= $message."\r\n";

// 添加附件
$attachment = chunk_split(base64_encode(file_get_contents('path/to/attachment')));
$body .= "--$boundary\r\n";
$body .= "Content-Type: application/octet-stream; name=\"attachment.txt\"\r\n";
$body .= "Content-Transfer-Encoding: base64\r\n";
$body .= "Content-Disposition: attachment\r\n\r\n";
$body .= $attachment."\r\n";

$body .= "--$boundary--";

// 发送邮件
if (mail($to, $subject, $body, $headers)) {
    echo "邮件发送成功";
} else {
    echo "邮件发送失败";
}
?>