前台界面：

后台代码：（不同的邮件主机可能会需要端口号）

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;using System.Net.Mail;using System.Net;using System.Threading;namespace Mail{    public partial class SendMail : Form    {        public SendMail()        {            InitializeComponent();        }        string[] FileNames        {            get;            set;        }        //发送邮件按钮        private void btnSend_Click(object sender, EventArgs e)        {            string to = tbTo.Text;            string from = "xxx@sina.com";            string subject = tbTheme.Text;            string content = @tbContent.Text;            MailMessage msg = new MailMessage(from, to, subject, content);            msg.IsBodyHtml = true;            //Thread threadMail = new Thread(new  ParameterizedThreadStart(SendEmail));            //threadMail.Start(msg);            ThreadPool.SetMaxThreads(0, 0);            ThreadPool.QueueUserWorkItem(new WaitCallback(SendEmail), msg);            btnSend.Enabled = false;        }        /// 
        /// 异步发邮件        /// 
        ///         private void SendEmail(object message)        {            SmtpClient client = new SmtpClient("smtp.sina.com");//有可能需要端口号            client.Credentials = new NetworkCredential("xxx@sina.com", "***");            MailMessage msg = message as MailMessage;            if (FileNames != null)            {                foreach (string fileName in FileNames)                {                    Attachment atta = new Attachment(fileName);                    msg.Attachments.Add(atta);                }            }            client.Send(msg);            this.Invoke(new ButtonEnableDelegate(EnableControl), new object[] { btnSend });            //Thread.CurrentThread.Abort();        }        delegate void ButtonEnableDelegate(Control control);        private void EnableControl(Control control)        {            control.Enabled = true;        }        //选择附件        private void btnAttach_Click(object sender, EventArgs e)        {            OpenFileDialog dia = new OpenFileDialog();            dia.Filter = "JPEG (*.jpg)|*.jpg|PNG (*.png)|*.png";            dia.RestoreDirectory = true;            dia.Multiselect = true;            if (dia.ShowDialog() == DialogResult.OK)            {                if (dia.FileNames.Length == 0)                {                    return;                }                this.FileNames = dia.FileNames;                this.tbAttach.Text = string.Empty;                foreach (string fileName in dia.FileNames)                {                    tbAttach.Text += fileName.Substring(fileName.LastIndexOf('\\')+1) + ";";                }            }        }    }}