C# 邮件发送，可根据需求修改为群发~

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Net;
using System.Net.Mail;
using System.Net.Mime;
using System.IO;

namespace SendMailExample
{
    /// <summary>
    /// 作者：Andrew
    /// Blog: http://blog.csdn.net/Andrew_wx     /// </summary>
    public partial class FormSendMail : Form
    {
        public FormSendMail()
        {
            InitializeComponent();
        }

        private void FormSendMail_Load(object sender, EventArgs e)
        {
            txtSmtpServer.Text = "smtp.qq.com";
            txtSend.Text = "heuandmei@qq.com";
            txtDisplayName.Text = "Andrew(王旭)";
            txtPassword.Text = "";//密码
            txtReceive.Text = "heuandmei@qq.com";
            txtTitle.Text = "发信测试";
            txtBody.Text = "This is a test(测试)";
            rbtnNoSSL.Checked = true;
        }

        private void btnAddFiles_Click(object sender, EventArgs e)
        {
            OpenFileDialog odlg = new OpenFileDialog();
            odlg.CheckFileExists = true;
            //只接收有效的文件名
            odlg.ValidateNames = true;
            //允许一次选择多个文件作为附件
            odlg.Multiselect = true;
            if (odlg.ShowDialog() == System.Windows.Forms.DialogResult.OK)
            {
                lstFiles.Items.AddRange(odlg.FileNames);
            }
            
        }

        private void btnSend_Click(object sender, EventArgs e)
        {
            this.Cursor = Cursors.WaitCursor;
            MailMessage mail = new MailMessage();
            mail.From = new MailAddress(
                txtSend.Text, txtDisplayName.Text, Encoding.UTF8);
            mail.To.Add(txtReceive.Text);
            mail.Subject = txtTitle.Text;
            mail.SubjectEncoding = Encoding.Default;
            mail.Body = txtBody.Text;
            mail.BodyEncoding = Encoding.Default;
            mail.IsBodyHtml = false;
            mail.Priority = MailPriority.Normal;
            //添加附件
            Attachment attachment = null;
            if (lstFiles.Items.Count > 0)
            {
                for (int i = 0; i < lstFiles.Items.Count; i++)
                {
                    string pathFileName = lstFiles.Items[i].ToString();
                    string extName = Path.GetExtension(pathFileName).ToLower();
                    //判断附件类型
                    if (extName == ".rar" || extName == ".zip")
                    {
                        attachment = new Attachment(pathFileName, MediaTypeNames.Application.Zip);
                    }
                    else
                    {
                        attachment = new Attachment(pathFileName, MediaTypeNames.Application.Octet);
                    }
                    ContentDisposition cd = attachment.ContentDisposition;
                    cd.CreationDate = File.GetCreationTime(pathFileName);
                    cd.ModificationDate = File.GetLastWriteTime(pathFileName);
                    cd.ReadDate = File.GetLastAccessTime(pathFileName);
                    mail.Attachments.Add(attachment);

                }
            }
            SmtpClient client = new SmtpClient();
            client.Host = txtSmtpServer.Text;
            client.Port = 25;
            //是否使用安全套接字层加密连接
            client.EnableSsl = rbtnUseSSL.Checked;
            //不使用默认凭证，注意此句必须放在 client.Credentials 的上面
            client.UseDefaultCredentials = false;
            client.Credentials = new NetworkCredential(txtSend.Text, txtPassword.Text);
            //邮件通过网络直接发送到服务器
            client.DeliveryMethod = SmtpDeliveryMethod.Network;
            try
            {
                client.Send(mail);
                MessageBox.Show("发送成功");
            }
            catch (SmtpException ex)
            {
                MessageBox.Show("发送失败：" + ex.Message);
            }
            catch (Exception ex)
            {
                MessageBox.Show("发送失败：" + ex.Message);
            }
            finally
            {
                mail.Dispose();
                client = null;
                this.Cursor = Cursors.Default;
            }
        }
    }
}