//要引用System.Web.Mail命名空间。引用该命名空间需要添加System.Web.Dll 

MailMessage mail = new MailMessage();
    mail.BodyFormat=MailFormat.Html;
    mail.From ="some@server";
    mail.To="some@server";//多个收件人之间用分号分割
    mail.Bcc="some@server";//密送
    mail.Cc=="some@server";//抄送
    mail.Subject = "this is mail subject";
    mail.Body ="this is mail body";
    mail.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpauthenticate", "1");
    mail.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendusername", "username"); 
    mail.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendpassword", "password);  
   
    System.Web.Mail.MailAttachment attachment=new MailAttachment("c:\\1.txt");
    mail.Attachments.Add(attachment);
    SmtpMail.SmtpServer = "10.2.140.100";
    SmtpMail.Send(mail);

------解决方案--------------------
楼上代码只能用于asp.net中,如果是c/s,只能封装到Web服务中,来调用实现.

------解决方案--------------------
http://blog.csdn.net/tianzhenjing/archive/2006/12/22/1454413.aspx

------解决方案--------------------
CS下同样有个发送接受邮件的命名控件using System.Net.Mail;
使用很简单，查查MSDN就行了

------解决方案--------------------
用JMAIL现成组件，什么都设计好了，设置好属性就能发了。不光能发还能收邮件。从pop3服务器里把邮件读取回来。打个广告，我资源里有，要的话去下吧。

------解决方案--------------------

http://topic.csdn.net/u/20080301/17/18568a4c-0c34-4bad-b0c5-08bc16a297ef.html

------解决方案--------------------
using System.Net.Mail;
using System.Net.Mime;
using System.Net;

  public static void CreateMessageWithAttachment(string server)
   {
       // Specify the file to be attached and sent.
       // This example assumes that a file named Data.xls exists in the
       // current working directory.
       string file = @"D:\asdf.txt";
       // Create a message and set up the recipients.
       MailMessage message = new MailMessage(
          "asdf@163.com",
          "asdf@163.com",
          "test",
          "no du");

       // Create  the file attachment for this e-mail message.
       Attachment data = new Attachment(file, MediaTypeNames.Application.Octet);
       // Add time stamp information for the file.
       ContentDisposition disposition = data.ContentDisposition;
       disposition.CreationDate = System.IO.File.GetCreationTime(file);
       disposition.ModificationDate = System.IO.File.GetLastWriteTime(file);
       disposition.ReadDate = System.IO.File.GetLastAccessTime(file);
       // Add the file attachment to this e-mail message.
       message.Attachments.Add(data);
       //Send the message.
       SmtpClient client = new SmtpClient(server);
       // Add credentials if the SMTP server requires them.
       //client.Credentials = CredentialCache.DefaultNetworkCredentials;
       client.UseDefaultCredentials = true;
       client.Credentials = new System.Net.NetworkCredential("username", "password");
       client.Send(message);
       // Display the values in the ContentDisposition for the attachment.
       ContentDisposition cd = data.ContentDisposition;
       Console.WriteLine("Content disposition");
       Console.WriteLine(cd.ToString());
       Console.WriteLine("File {0}", cd.FileName);