可以使用GMail发mail，据说G官方限制一天内同一封邮件最多发送到500个联系人。代码如下：

namespace ConsoleApplication9
{
    class Program
    {
        static void Main(string[] args)
        {
            var from = new MailAddress("asdf@112.com", "cary"); 
            var to = new MailAddress("chenghui921@126.com", "toname");
            var message = new MailMessage(from, to);
            message.Subject = "The subject";
            message.Body = "The message body";
            message.IsBodyHtml = true;
            var host = "smtp.gmail.com";
            var client = new SmtpClient(host, 587);
            client.EnableSsl = true;
            client.Credentials = new NetworkCredential("<<your username>>", "<<your password>>");
            client.Send(message);
            Console.ReadLine();        
        }          
    }

}

本文转自生鱼片博客园博客，原文链接：http://www.cnblogs.com/carysun/archive/2009/11/12/sendmail-by-gmail.html，如需转载请自行联系原作者