C#实现SMTP服务器，使用TCP命令实现，功能比较完善

using System;
using System.Text;
using System.IO;
using System.Net;
using System.Net.Sockets;
using System.Collections;

namespace SkyDev.Web.Mail
{
public enum MailFormat{Text,HTML};
public enum MailPriority{Low=1,Normal=3,High=5};

#region Class mailAttachments
public class MailAttachments
{
private const int MaxAttachmentNum=10;
private IList _Attachments;

public MailAttachments()
{
_Attachments=new ArrayList();
}

public string this[int index]
{
get { return (string)_Attachments[index];}
}
/// <summary>
/// 添加邮件附件
/// </summary>
/// <param name="FilePath">附件的绝对路径</param>
public void Add(params string[] filePath)
{
if(filePath==null)
{
throw(new ArgumentNullException("非法的附件"));
}
else
{
for(int i=0;i<filePath.Length;i++)
{
Add(filePath[i]);
}
}
}

/// <summary>
/// 添加一个附件,当指定的附件不存在时，忽略该附件，不产生异常。
/// </summary>
/// <param name="filePath">附件的绝对路径</param>
public void Add(string filePath)
{
//当附件存在时才加入,否则忽略
if (System.IO.File.Exists(filePath))
{
if (_Attachments.Count<MaxAttachmentNum)
{
_Attachments.Add(filePath);
}
}
}

public void Clear()//清除所有附件
{
_Attachments.Clear();
}

public int Count//获取附件个数
{
get { return _Attachments.Count;}
}

}
#endregion//end Class mailAttachments



#region Class MailMessage
/// <summary>
/// MailMessage 表示SMTP要发送的一封邮件的消息。
/// </summary>
public class MailMessage
{
private const int MaxRecipientNum=10;
public MailMessage()
{
_Recipients=new ArrayList();//收件人列表
_Attachments=new MailAttachments();//附件
_BodyFormat=MailFormat.Text;//缺省的邮件格式为Text
_Priority=MailPriority.Normal;
_Charset="GB2312";
}

/// <summary>
/// 设定语言代码，默认设定为GB2312，如不需要可设置为""
/// </summary>
public string Charset
{
get { return _Charset;}
set { _Charset=value;}
}

public string From
{
get{ return _From;}
set { _From=value;}
}

public string FromName
{
get { return _FromName;}
set { _FromName=value;}
}
public string Body
{
get { return _Body;}
set { _Body=value;}
}

public string Subject
{
get { return _Subject;}
set { _Subject=value;}
}

public MailAttachments Attachments
{
get {return _Attachments;}
set { _Attachments=value;}
}

public MailPriority Priority
{
get { return _Priority;}
set { _Priority=value;}
}

public IList Recipients
{
get { return _Recipients;}
}
/// <summary>
/// 增加一个收件人地址
/// </summary>
/// <param name="recipient">收件人的Email地址</param>
public void AddRecipients(string recipient)
{
//先检查邮件地址是否符合规范
if (_Recipients.Count<MaxRecipientNum)
{
_Recipients.Add(recipient);//增加到收件人列表
}
}

public void AddRecipients(params string[] recipient)
{
if (recipient==null)
{
throw (new ArgumentException("收件人不能为空."));
}
else
{
for (int i=0;i<recipient.Length;i++)
{
AddRecipients(recipient[i]);
}
}
}

public MailFormat BodyFormat
{
set { _BodyFormat=value;}
get { return _BodyFormat;}
}

private string _From;//发件人地址
private string _FromName;//发件人姓名
private IList _Recipients;//收件人
private MailAttachments _Attachments;//附件
private string _Body;//内容
private string _Subject;//主题
private MailFormat _BodyFormat;//邮件格式
private string _Charset="GB2312";//字符编码格式
private MailPriority _Priority;//邮件优先级
}
#endregion


#region Class SmtpMail
public class SmtpServerHelper
{
private string CRLF="/r/n";//回车换行

/// <summary>
/// 错误消息反馈
/// </summary>
private string errmsg;

/// <summary>
/// TcpClient对象，用于连接服务器
/// </summary>
private TcpClient tcpClient;

/// <summary>
/// NetworkStream对象
/// </summary>
private NetworkStream networkStream;

/// <summary>
/// 服务器交互记录
/// </summary>
private string logs="";

/// <summary>
/// SMTP错误代码哈希表
/// </summary>
private Hashtable ErrCodeHT = new Hashtable();

/// <summary>
/// SMTP正确代码哈希表
/// </summary>
private Hashtable RightCodeHT = new Hashtable();

public SmtpServerHelper()
{
SMTPCodeAdd();//初始化SMTPCode
}

~SmtpServerHelper()
{
networkStream.Close();
tcpClient.Close();
}

/// <summary>
/// 将字符串编码为Base64字符串
/// </summary>
/// <param name="str">要编码的字符串</param>
private string Base64Encode(string str)
{
byte[] barray;
barray=Encoding.Default.GetBytes(str);
return Convert.ToBase64String(barray);
}

/// <summary>
/// 将Base64字符串解码为普通字符串
/// </summary>
/// <param name="str">要解码的字符串</param>
private string Base64Decode(string str)
{
byte[] barray;
barray=Convert.FromBase64String(str);
return Encoding.Default.GetString(barray);
}

/// <summary>
/// 得到上传附件的文件流
/// </summary>
/// <param name="FilePath">附件的绝对路径</param>
private string GetStream(string FilePath)
{
//建立文件流对象
System.IO.FileStream FileStr=new System.IO.FileStream(FilePath,System.IO.FileMode.Open);
byte[] by=new byte[System.Convert.ToInt32(FileStr.Length)];
FileStr.Read(by,0,by.Length);
FileStr.Close();
return(System.Convert.ToBase64String(by));
}

/// <summary>
/// SMTP回应代码哈希表
/// </summary>
private void SMTPCodeAdd()
{
//[RFC 821 4.2.1.]
/*
4.2.2. NUMERIC ORDER LIST OF REPLY CODES

211 System status, or system help reply
214 Help message
[Information on how to use the receiver or the meaning of a
particular non-standard command; this reply is useful only
to the human user]
220 <domain> Service ready
221 <domain> Service closing transmission channel
250 Requested mail action okay, completed
251 User not local; will forward to <forward-path>

354 Start mail input; end with <CRLF>.<CRLF>

421 <domain> Service not available,
closing transmission channel
[This may be a reply to any command if the service knows it
must shut down]
450 Requested mail action not taken: mailbox unavailable
[E.g., mailbox busy]
451 Requested action aborted: local error in processing
452 Requested action not taken: insufficient system storage

500 Syntax error, command unrecognized
[This may include errors such as command line too long]
501 Syntax error in parameters or arguments
502 Command not implemented
503 Bad sequence of commands
504 Command parameter not implemented
550 Requested action not taken: mailbox unavailable
[E.g., mailbox not found, no access]
551 User not local; please try <forward-path>
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed
[E.g., mailbox syntax incorrect]
554 Transaction failed

*/

ErrCodeHT.Add("421","服务未就绪，关闭传输信道");
ErrCodeHT.Add("432","需要一个密码转换");
ErrCodeHT.Add("450","要求的邮件操作未完成，邮箱不可用（例如，邮箱忙）");
ErrCodeHT.Add("451","放弃要求的操作；处理过程中出错");
ErrCodeHT.Add("452","系统存储不足，要求的操作未执行");
ErrCodeHT.Add("454","临时认证失败");
ErrCodeHT.Add("500","邮箱地址错误");
ErrCodeHT.Add("501","参数格式错误");
ErrCodeHT.Add("502","命令不可实现");
ErrCodeHT.Add("503","服务器需要SMTP验证");
ErrCodeHT.Add("504","命令参数不可实现");
ErrCodeHT.Add("530","需要认证");
ErrCodeHT.Add("534","认证机制过于简单");
ErrCodeHT.Add("538","当前请求的认证机制需要加密");
ErrCodeHT.Add("550","要求的邮件操作未完成，邮箱不可用（例如，邮箱未找到，或不可访问）");
ErrCodeHT.Add("551","用户非本地，请尝试<forward-path>");
ErrCodeHT.Add("552","过量的存储分配，要求的操作未执行");
ErrCodeHT.Add("553","邮箱名不可用，要求的操作未执行（例如邮箱格式错误）");
ErrCodeHT.Add("554","传输失败");


/*
211 System status, or system help reply
214 Help message
[Information on how to use the receiver or the meaning of a
particular non-standard command; this reply is useful only
to the human user]
220 <domain> Service ready
221 <domain> Service closing transmission channel
250 Requested mail action okay, completed
251 User not local; will forward to <forward-path>

354 Start mail input; end with <CRLF>.<CRLF>
*/

RightCodeHT.Add("220","服务就绪");
RightCodeHT.Add("221","服务关闭传输信道");
RightCodeHT.Add("235","验证成功");
RightCodeHT.Add("250","要求的邮件操作完成");
RightCodeHT.Add("251","非本地用户，将转发向<forward-path>");
RightCodeHT.Add("334","服务器响应验证Base64字符串");
RightCodeHT.Add("354","开始邮件输入，以<CRLF>.<CRLF>结束");

}

/// <summary>
/// 发送SMTP命令
/// </summary>
private bool SendCommand(string str)
{
byte[]WriteBuffer;
if(str==null||str.Trim()==String.Empty)
{
return true;
}
logs+=str;
WriteBuffer = Encoding.Default.GetBytes(str);
try
{
networkStream.Write(WriteBuffer,0,WriteBuffer.Length);
}
catch
{
errmsg="网络连接错误";
return false;
}
return true;
}

/// <summary>
/// 接收SMTP服务器回应
/// </summary>
private string RecvResponse()
{
int StreamSize;
string Returnvalue = String.Empty;
byte[] ReadBuffer = new byte[1024] ;
try
{
StreamSize=networkStream.Read(ReadBuffer,0,ReadBuffer.Length);
}
catch
{
errmsg="网络连接错误";
return "false";
}

if (StreamSize==0)
{
return Returnvalue ;
}
else
{
Returnvalue = Encoding.Default.GetString(ReadBuffer).Substring(0,StreamSize);
logs+=Returnvalue+this.CRLF;
return Returnvalue;
}
}

/// <summary>
/// 与服务器交互，发送一条命令并接收回应。
/// </summary>
/// <param name="str">一个要发送的命令</param>
/// <param name="errstr">如果错误，要反馈的信息</param>
private bool Dialog(string str,string errstr)
{
if(str==null||str.Trim()==string.Empty)
{
return true;
}
if(SendCommand(str))
{
string RR=RecvResponse();
if(RR=="false")
{
return false;
}
//检查返回的代码，根据[RFC 821]返回代码为3位数字代码如220
string RRCode=RR.Substring(0,3);
if(RightCodeHT[RRCode]!=null)
{
return true;
}
else
{
if(ErrCodeHT[RRCode]!=null)
{
errmsg+=(RRCode+ErrCodeHT[RRCode].ToString());
errmsg+=CRLF;
}
else
{
errmsg+=RR;
}
errmsg+=errstr;
return false;
}
}
else
{
return false;
}
}


/// <summary>
/// 与服务器交互，发送一组命令并接收回应。
/// </summary>

private bool Dialog(string[] str,string errstr)
{
for(int i=0;i<str.Length;i++)
{
if(!Dialog(str[i],""))
{
errmsg+=CRLF;
errmsg+=errstr;
return false;
}
}

return true;
}


//连接服务器
private bool Connect(string smtpServer,int port)
{
//创建Tcp连接
try
{
tcpClient=new TcpClient(smtpServer,port);
}
catch(Exception e)
{
errmsg=e.ToString();
return false;
}
networkStream=tcpClient.GetStream();

//验证网络连接是否正确
if(RightCodeHT[RecvResponse().Substring(0,3)]==null)
{
errmsg="网络连接失败";
return false;
}
return true;
}

private string GetPriorityString(MailPriority mailPriority)
{
string priority="Normal";
if (mailPriority==MailPriority.Low)
{
priority="Low";
}
else if (mailPriority==MailPriority.High)
{
priority="High";
}
return priority;
}

/// <summary>
/// 发送电子邮件，SMTP服务器不需要身份验证
/// </summary>
/// <param name="smtpServer"></param>
/// <param name="port"></param>
/// <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;
/