gmail的smtp采用了ssl连接: Outgoing Mail (SMTP) Server - requires TLS: smtp.gmail.com (use authentication) Use Authentication: Yes Use STARTTLS: Yes (some clients call this SSL) Port: 465 or 587 代码：

using System;
using System.Collections.Generic;
using System.Text;
using System.Net;
using System.Net.Mail;

namespace GMailSend
{
    class Program
    {
        static void Main(string\[\] args)
        {
            try
            {
                Program prg = new Program();
                prg.Host = "smtp.gmail.com";
                prg.SmtpUsername = "zhangshanyou@gmail.com";
                prg.SmtpPassword = "xxxxxxxx";
                prg.Port = 587;
                prg.Send("zhangshanyou@gmail.com","33235911@qq.com",  "test", "test", null, null);
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);

            }
            Console.Read();
        }

        private string \_host;

        public string Host
        {
            get { return \_host; }
            set { \_host = value; }
        }
        private int \_port;

        public int Port
        {
            get { return \_port; }
            set { \_port = value; }
        }
        private string \_smtpUsername;

        public string SmtpUsername
        {
            get { return \_smtpUsername; }
            set { \_smtpUsername = value; }
        }
        private string \_smtpPassword;

        public string SmtpPassword
        {
            get { return \_smtpPassword; }
            set { \_smtpPassword = value; }
        }

        public void Send(string from, string to, string subject, string body, string\[\] cc, string\[\] bcc)
        {
            // Create mail message
            MailMessage message = new MailMessage(from, to, subject, body);
            message.BodyEncoding = Encoding.GetEncoding(936);

            if (cc != null && cc.Length > 0)
            {
                foreach (string ccAddress in cc)
                {
                    message.CC.Add(new MailAddress(ccAddress));
                }
            }
            if (bcc != null && bcc.Length > 0)
            {
                foreach (string bccAddress in bcc)
                {
                    message.Bcc.Add(new MailAddress(bccAddress));
                }
            }

            // Send email
            SmtpClient client = new SmtpClient(this.\_host, this.\_port);
            if (!String.IsNullOrEmpty(this.\_smtpUsername) && !String.IsNullOrEmpty(this.\_smtpPassword))
            {
                client.Credentials = new NetworkCredential(this.\_smtpUsername, this.\_smtpPassword);
            }
            client.EnableSsl = true;

            client.Send(message);

        }

    }
}