WinForm窗体代码如下：

<span style="font-size:14px;">using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Linq;

using System.Text;

using System.Windows.Forms;

<span style="color:#ff0000;">//要添加的引用如下</span>

using System.Net.Mail;

using System.Net.Mime;

using System.Timers;

using System.IO;

namespace NovelCS {

    public partial class Form3 : Form {

        public Form3() {

            InitializeComponent();

        }

        private void Form3_Load(object sender, EventArgs e) {

            //一些常见的发送邮件服务器地址 一般都是 stmp.**.com   **=qq 或 126 或163 或 gmail 有例外的，具体问度娘

            string senderServerIp = "smtp.qq.com";

            string toMailAddress = "1449740504@qq.com";

            string fromMailAddress = "1449740504@qq.com";

            string subjectInfo = "My First Email";

            string bodyInfo =@"Hello Jackerson, \r\n

                                        This is my first testing e_mail.But what all thess is from  an author named Eric Sun.

                                        Thank you,Eric Sun.";

            string mailUsername = "1449740504";

            string mailPassword = "******"; //发送人邮箱的密码

            string mailPort = "25";   //邮箱端口号

            string attachPath = "D:\\jsj.txt; D:\\jsj2.txt";//附件本地地址

            MyEmail email = new MyEmail(senderServerIp, toMailAddress, fromMailAddress, subjectInfo, bodyInfo, mailUsername, mailPassword, mailPort, false, false);

            email.AddAttachments(attachPath);

            email.Send();

        }

    }

    public class MyEmail {

        private MailMessage me;//主要处理发送邮件的内容，例：收发件人地址、标题、主体、图片

        private SmtpClient sc;   //主要处理用smtp发送邮件的配置信息，例：发送邮件服务器地址、发送端口号、验证方式

        private int mSenderPort;                         //发送邮件所用的端口号（htmp协议默认为25）

        private string mSenderServerHost;          //发件箱的邮件服务器地址（IP形式或字符串形式均可）

        private string mSenderUserName;           //发件箱的用户名（即@符号前面的字符串，例如：hello@163.com，用户名为：hello）

        private string mSenderPassword;             //发件箱的密码

        private bool mEnableSsl;                         //是否对邮件内容进行socket层加密传输

        private bool mEnablePwdAuthentication;  //是否对发件人邮箱进行密码验证

        ///<summary>

        /// <strong><span style="color:#ff0000;">构造函数</span></strong>

        ///</summary>

        ///<param name="server">发件箱的邮件服务器地址</param>

        ///<param name="toMail">收件人地址（可以是多个收件人，程序中是以“;"进行区分的）</param>

        ///<param name="fromMail">发件人地址</param>

        ///<param name="subject">邮件标题</param>

        ///<param name="emailBody">邮件内容（可以以html格式进行设计）</param>

        ///<param name="username">发件箱的用户名（即@符号前面的字符串，例如：hello@163.com，用户名为：hello）</param>

        ///<param name="password">发件人邮箱密码</param>

        ///<param name="port">发送邮件所用的端口号（htmp协议默认为25）</param>

        ///<param name="sslEnable">true表示对邮件内容进行socket层加密传输，false表示不加密</param>

        ///<param name="pwdCheckEnable">true表示对发件人邮箱进行密码验证，false表示不对发件人邮箱进行密码验证</param>

        public MyEmail(string server, string toMailAddress, string fromMailAddress, string emailTitle, string emailBody, string username, string password, string port, bool sslEnable, bool pwdCheckEnable) {

            try {

                me = new MailMessage();

                me.To.Add(toMailAddress);

                me.From = new MailAddress(fromMailAddress);

                me.Subject = emailTitle;

                me.Body = emailBody;

                me.IsBodyHtml = true;

                me.BodyEncoding = System.Text.Encoding.UTF8;

                me.Priority = MailPriority.Normal;

                this.mSenderServerHost = server;

                this.mSenderUserName = username;

                this.mSenderPassword = password;

                this.mSenderPort = Convert.ToInt32(port);

                this.mEnableSsl = sslEnable;

                this.mEnablePwdAuthentication = pwdCheckEnable;

            } catch (Exception ex) {

                MessageBox.Show(ex.ToString());

            }

        }

        ///<summary>

        /// <strong><span style="color:#ff0000;">添加附件</span></strong>

        ///</summary>

        ///<param name="attachmentsPath">附件的路径集合，以分号分隔</param>

        public void AddAttachments(string attachmentsPath) {

            try {

                string[] path = attachmentsPath.Split(';'); //以什么符号分隔可以自定义

                Attachment data;

                ContentDisposition disposition;

                for (int i = 0; i < path.Length; i++) {

                    data = new Attachment(path[i], MediaTypeNames.Application.Octet);

                    disposition = data.ContentDisposition;

                    disposition.CreationDate = File.GetCreationTime(path[i]);

                    disposition.ModificationDate = File.GetLastWriteTime(path[i]);

                    disposition.ReadDate = File.GetLastAccessTime(path[i]);

                    me.Attachments.Add(data);

                }

            } catch (Exception ex) {

                MessageBox.Show(ex.ToString());

            }

        }

        ///<summary>

        /// <strong><span style="color:#ff0000;">邮件的发送</span></strong>

        ///</summary>

        public void Send() {

            try {

                if (me != null) {

                    sc = new SmtpClient();

                    //mSmtpClient.Host = "smtp." + mMailMessage.From.Host;

                    sc.Host = this.mSenderServerHost;

                    sc.Port = this.mSenderPort;

                    sc.UseDefaultCredentials = false;

                    sc.EnableSsl = this.mEnableSsl;

                    if (this.mEnablePwdAuthentication) {

                        System.Net.NetworkCredential nc = new System.Net.NetworkCredential(this.mSenderUserName, this.mSenderPassword);

                        //mSmtpClient.Credentials = new System.Net.NetworkCredential(this.mSenderUsername, this.mSenderPassword);

                        //NTLM: Secure Password Authentication in Microsoft Outlook Express

                        sc.Credentials = nc.GetCredential(sc.Host, sc.Port, "NTLM");

                    } else {

                        sc.Credentials = new System.Net.NetworkCredential(this.mSenderUserName, this.mSenderPassword);

                    }

                    sc.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network;

                    sc.Send(me);

                }

            } catch (Exception ex) {

                Console.WriteLine(ex.ToString());

            }

        }

    }      

}</span>

备注：

(1)经过测试，确实可以使用。

(2)本文出处：Eric Sun(http://www.cnblogs.com/mingmingruyuedlut/archive/2011/10/14/2212255.html)