Send mail without specifying an SMTP server

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Overview

Quite frequently, you hear people asking how they can send an email from their applications to a specific email address without prompting the user for a relaying SMTP server. The most commonly suggested solution is to use MAPI and get a list of SMTP servers from the user's registry, but this assumes that the user has configured a MAPI compliant email client on his machine. I always reply with my solution of querying the MX record for the target domain and then SMTP-chatting to that server directly - it's easy to do, it works on any machine and it's also the fastest way to send email (minimum number of SMTP hops).

Starting with VC++ 7, ATL includes the CSMTPConnection class which lets you send mail, provided you give it an SMTP server to connect to. I derived a class (CSMTPConnection2) from CSMTPConnection which lets you pass in the target domain name directly and the class queries the MX records using the default DNS on the client machine, gets a list of SMTP servers (depending on the number of MX records returned) and connects to the first SMTP server that works okay. The only visible change is to the Connect method, and even for the Connect method, the method signature remains the same. The first parameter (an LPCTSTR) now represents the target domain name instead of the SMTP hostname. So it should be pretty easy to modify your existing code. Just change all references to CSMTPConnection in your code to CSMTPConnection2 and you are done.

Using the class

• #include the header file for the class

  #include "smtpconnection2.h"

  The header file includes a #pragma linker directive to include Dnsapi.lib

• Use the CSMTPConnection2 class just as you would use the CSMTPConnection class (it's assumed that CoInitialize has been called prior to invoking and using this class)

  CMimeMessage msg;
  msg.SetSender("nish@somedomain.com");
  msg.SetSenderName("Nishter");
  msg.AddRecipient("someone@vsnl.com");
  
  //Optional
  msg.SetSubject("Hello World");
  msg.AddText("Hmmm, this should work fine!");
  //msg.AttachFile("some file path");
  
  CSMTPConnection2 conn;
  //You need to specify the domain name of the recipient email address
  if(conn.Connect("vsnl.com"))
  {
      if( conn.SendMessage(msg) == TRUE )
          std::cout << "Mail sent successfully" << std::endl;
      conn.Disconnect();    
  }
  

Class requirements

• You'll need VC++ 7 or above (for the ATL classes)

• This code will run only on Windows 2000 or later (because of the DNS API that's used to query the MX record)

• You do not need MFC (the CString that's used is shared between MFC/ATL and I specifically used CSimpleArray instead of an MFC array class)

Limitation

Note that, because the class looks up the MX record and SMTPs directly into the mail server for each domain name, you cannot use this class to send mails to multiple-domains in one go.

Source listings

Header file

/*
    Copyright(C) Nishant Sivakumar (nish#voidnish.com)
    Please maintain this copyright notice if you distribute this file
    in original or modified form.
*/

#pragma once
#include <atlsmtpconnection.h>
#include <Windns.h>

#pragma comment(lib,"Dnsapi.lib")

class CSMTPConnection2 : public CSMTPConnection
{
public:    
    BOOL Connect(LPCTSTR lpszHostDomain, DWORD dwTimeout = 10000) throw();
private:
    void _GetSMTPList(LPCTSTR lpszHostDomain, CSimpleArray<CString>& arrSMTP);
}; 

Cpp file

/*
    Copyright(C) Nishant Sivakumar (nish#voidnish.com)
    Please maintain this copyright notice if you distribute this file
    in original or modified form.
*/

#include "StdAfx.h"
#include "smtpconnection2.h"

BOOL CSMTPConnection2::Connect(LPCTSTR lpszHostDomain, 
                               DWORD dwTimeout /*= 10000*/) throw()
{
    CSimpleArray<CString> arrSMTP;
    _GetSMTPList(lpszHostDomain, arrSMTP);
    for(int i=0; i<arrSMTP.GetSize(); i++)
    {
        if(CSMTPConnection::Connect(arrSMTP[i], dwTimeout) == TRUE)
            return TRUE;
    }
    return FALSE;
}

void CSMTPConnection2::_GetSMTPList(LPCTSTR lpszHostDomain, 
                                   CSimpleArray<CString>& arrSMTP)
{
    PDNS_RECORD pRec = NULL;
    if(DnsQuery(lpszHostDomain, DNS_TYPE_MX, DNS_QUERY_STANDARD,
        NULL, &pRec, NULL) == ERROR_SUCCESS)
    {
        PDNS_RECORD pRecOrig = pRec;
        while(pRec)
        {
            if(pRec->wType == DNS_TYPE_MX)
                arrSMTP.Add(pRec->Data.MX.pNameExchange);
            pRec = pRec->pNext;
        }
        DnsRecordListFree(pRecOrig,DnsFreeRecordList);
    }
} 

Conclusion

As usual, feedback is welcome and appreciated whether it's constructive or not.

This article, along with any associated source code and files, is licensed under The Microsoft Public License (Ms-PL)