无法使用C#向多个地址/收件人发送电子邮件

using System;
using System.Net.Mail;

class Program
{
    static void Main(string[] args)
    {
        // 创建MailMessage对象
        MailMessage mail = new MailMessage();

        // 设置发件人
        mail.From = new MailAddress("sender@example.com");

        // 添加收件人
        mail.To.Add("recipient1@example.com");
        mail.To.Add("recipient2@example.com");
        // 可以继续添加更多的收件人

        // 设置邮件主题和内容
        mail.Subject = "Hello from C#";
        mail.Body = "This is a test email.";

        // 创建SmtpClient对象并设置SMTP服务器信息
        SmtpClient smtpClient = new SmtpClient("smtp.example.com", 25);
        smtpClient.Credentials = new System.Net.NetworkCredential("username", "password");

        try
        {
            // 发送邮件
            smtpClient.Send(mail);
            Console.WriteLine("Email sent successfully.");
        }
        catch (Exception ex)
        {
            Console.WriteLine("Failed to send email. Error message: " + ex.Message);
        }
    }
}