C#实现SMTP服务器，使用TCP命令实现，功能比较完善[5]

  /// <param name="mailmessage"></param>
  /// <returns></returns>
  public  bool sendemail(string smtpserver,int port,mailmessage mailmessage)
  {
   return sendemail(smtpserver,port,false,"","",mailmessage);
  }

  /// <summary>
  /// 发送电子邮件，smtp服务器需要身份验证
  /// </summary>
  /// <param name="smtpserver"></param>
  /// <param name="port"></param>
  /// <param name="username"></param>
  /// <param name="password"></param>
  /// <param name="mailmessage"></param>
  /// <returns></returns>
  public bool sendemail(string smtpserver,int port,string username,string password,mailmessage mailmessage)
  {
   return sendemail(smtpserver,port,false,username,password,mailmessage);
  }

  private  bool sendemail(string smtpserver,int port,bool esmtp,string username,string password,mailmessage mailmessage)
  { 
   if (connect(smtpserver,port)==false)//测试连接服务器是否成功
    return false;

   string priority=getprioritystring(mailmessage.priority);
   bool html=(mailmessage.bodyformat==mailformat.html);
   
   string[] sendbuffer;
   string sendbufferstr;

   //进行smtp验证，现在大部分smtp服务器都要认证
   if(esmtp)
   {
    sendbuffer=new string[4];
    sendbuffer[0]="ehlo " + smtpserver + crlf;
    sendbuffer[1]="auth login" + crlf;
    sendbuffer[2]=base64encode(username) + crlf;
    sendbuffer[3]=base64encode(password) + crlf;
    if(!dialog(sendbuffer,"smtp服务器验证失败，请核对用户名和密码。"))
     return false;
   }
   else
   {//不需要身份认证
    sendbufferstr="helo " + smtpserver + crlf;
    if(!dialog(sendbufferstr,""))
     return false;
   }

   //发件人地址
   sendbufferstr="mail from:<" + mailmessage.from + ">" + crlf;
   if(!dialog(sendbufferstr,"发件人地址错误，或不能为空"))
    return false;

   //收件人地址
   sendbuffer=new string[mailmessage.recipients.count];
   for(int i=0;i<mailmessage.recipients.count;i++)
   {
    sendbuffer[i]="rcpt to:<" +(string)mailmessage.recipients[i] +">" + crlf;
   }
   if(!dialog(sendbuffer,"收件人地址有误"))
    return false;

   /*
     sendbuffer=new string[10];
     for(int i=0;i<recipientbcc.count;i++)
     {

   sendbuffer[i]="rcpt to:<" + recipientbcc[i].tostring() +">" + crlf;

     }

     if(!dialog(sendbuffer,"密件收件人地址有误"))
   return false;
   */

   sendbufferstr="data" + crlf;
   if(!dialog(sendbufferstr,""))
    return false;

   //发件人姓名
   sendbufferstr="from:" + mailmessage.fromname + "<" +mailmessage.from +">" +crlf;

   //if(replyto.trim()!="")
   //{
   // sendbufferstr+="reply-to: " + replyto + crlf;
   //}

   //sendbufferstr+="to:" + toname + "<" + recipient[0] +">" +crlf;
   //至少要有一个收件人
   if (mailmessage.recipients.count==0)
   {
    return false;
   }
   else
   {
    sendbufferstr += "to:=?"+mailmessage.charset.toupper()+"?b?"+
     base64encode((string)mailmessage.recipients[0])+"?="+"<"+(string)mailmessage.recipients[0]+">"+crlf;
   }
 
   //sendbufferstr+="cc:";
   //for(int i=0;i<recipient.count;i++)
   //{
   // sendbufferstr+=recipient[i].tostring() + "<" + recipient[i].tostring() +">,";
   //}
   //sendbufferstr+=crlf;

   sendbufferstr+=