Question

is there a simple way to look up if a domain has a MX record or not using Delphi? I have a list of emails that I wish to verify work, I want to check each of the domains and see if a MX server even exists.

Thanks.

Edit: The email addresses I have are all from bounced email messages of error code: 5.4.0. But too many servers don’t follow any standards and 5.4.0 error code itself can mean too much. I don’t want to just remove all the email addresses found with that error code erroraneously, so I figure a better way is to first check if the domain or mx record don’t exist and remove those for sure.

Answer 1

It is actually good to have an e-mail checker. If nothing else you can clean you e-mail base and avoid sending over and over to non existing mails. Or you can use it as means of verifying user mails when they sign on to your application.

Here is a part of the code in my mail checking class.

procedure TMailValidator.ResolveEmailAddress(const Address: TEMailAddress; const DNSServer: string);
var
  I: Integer;
  MXEmpty: Boolean;
  DomainName: string;
  DNSResolver: TIdDNSResolver;
begin
  DNSResolver := TIdDNSResolver.Create(nil);
  try
    DomainName := StrAfter('@', string(Address));
    MXEmpty := True;

    DNSResolver.Host := DNSServer;
    {$IFNDEF IT_UseIndy9}
      DNSResolver.QueryType := [qtMx];
    {$ELSE}
      DNSResolver.QueryRecords := [qtMx];
    {$ENDIF}  // IT_UseIndy9
    try
      {$IFNDEF IT_UseIndy9}
        DNSResolver.WaitingTime := FDNSResolveTimeout;
      {$ELSE}
        DNSResolver.ReceiveTimeout := FDNSResolveTimeout;
      {$ENDIF}  // IT_UseIndy10
      DNSResolver.Resolve(DomainName);

      for I := 0 to DNSResolver.QueryResult.Count - 1 do
      begin
        if DNSResolver.QueryResult[I].RecType = qtMX then
        begin
          MXEmpty := False;
          CheckEmailAddress(Address, TMXRecord(DNSResolver.QueryResult[I]).ExchangeServer);

          // were we successfull
          if CheckSMTPExitErrorCode then
            Exit;
        end;
      end;

      // check for servers flag
      if FFoundMailServer then
      begin
        SendLogMessage(Format('Address "%s" is not valid on domain "%s"', [Address, DNSServer]));
        SetLastError(cUserErrorCodeBase + 5);
      end
      else
      begin
        if MXEmpty then
        begin
          SendLogMessage(Format('No valid mail(MX) server could be found for domain "%s"', [DomainName]));
          CheckEmailAddress(Address, DomainName);
        end
        else
        begin
          SendLogMessage(Format('Mail server did not respond on domain "%s"', [DomainName]));
          SetLastError(cUserErrorCodeBase + 3);
        end;
      end;
    except
      on E: Exception do
      begin
        SendLogMessage(Format('Address "%s" validation failed for domain "%s": %s', [Address,
                                                                                     DomainName,
                                                                                     E.Message]));
        SetLastError(cUserErrorCodeBase + 4, E.Message);
      end;
    end;
  finally
    DNSResolver.Free;
  end;
end;


procedure TMailValidator.CheckEmailAddress(const Address: TEMailAddress; const MailServer: string);
var
  SMTP: TIdSMTP;
begin
  SendLogMessage(Format('Validating address "%s" on server "%s"', [Address, MailServer]));

  if (FCheckStep = csAddress) or (FCheckStep = csDomain) then
  begin
    // finish if flags in [FLAG_CheckLocal, FLAG_CheckDomain]
    SendLogMessage(Format('Address "%s" successfuly validated.', [Address]));
    Exit;
  end;

  SMTP := TIdSMTP.Create(nil);
  try
    FCurrentStep := csMailBox;
    try
      SMTP.ReadTimeout := FSMTPReadTimeout;
      {$IFNDEF IT_UseIndy9}
        SMTP.AuthType := satNone;
      {$ELSE}
        SMTP.AuthenticationType := atNone;
      {$ENDIF}  // IT_UseIndy9
      SMTP.Host := MailServer;
      SMTP.Port := 25;

      SMTP.Connect;
      try
        FFoundMailServer := True;

        try
          SMTP.SendCmd('Helo ' + FQueryingServer, 250 );
          SMTP.SendCmd('Rset');
          SMTP.SendCmd('Mail from:<' + string(Address) + '>', 250);
          SMTP.SendCmd('RCPT to:<' + string(Address) + '>', [250, 251] );

          SendLogMessage(Format('Address "%s" successfuly validated on server "%s".', [FEMailAddress,
                                                                                       MailServer]));
        except
          on E: Exception do
          begin
            SendLogMessage(Format('Address "%s" validation failed on server "%s": %s', [Address,
                                                                                        MailServer,
                                                                                        E.Message]));
            SetLastError(SMTP.LastCmdResult.NumericCode, E.Message);
            Exit;
          end;
        end
      finally
        SMTP.Disconnect;
      end;
    except
      // handle all other exceptions
      on E: Exception do
      begin
        SendLogMessage(Format('CheckMail [%s] : Failure (Server) "%s" [%s]', [Address,
                                                                              MailServer,
                                                                              E.Message]));
        SetLastError(Max(cUserErrorCodeBase + 6, SMTP.LastCmdResult.NumericCode), E.Message);
      end;
    end;
  finally
    SMTP.Free;
  end;
end;

Basically you do it in three steps:

1. Check the mail syntax.
2. Check the domain and validate MX server
3. Validate the user mailbox