function TForm1.CheckOfMailServer(DNSServer, Adresse,
  MailIPAdress: String): Boolean;
var
   idDNSResolver: TIdDNSResolver;
  i: Integer;
  a: TARecord;
begin
   Result := False;
   idDNSResolver := TIdDNSResolver.Create(self);

  try
     idDNSResolver.QueryResult.Clear;
     idDNSResolver.Host := DNSServer;
     idDNSResolver.QueryType := [qtA];

    try
        idDNSResolver.Resolve(Adresse);
      except
         idDNSResolver.QueryResult.Clear;
     end;

     if idDNSResolver.QueryResult.Count <> 0 then
     begin
        for i := 0 to Pred(idDNSResolver.QueryResult.Count) do
       begin
          if idDNSResolver.QueryResult.Items[i] is TARecord then
         begin
            a := (idDNSResolver.QueryResult.Items[i] as TARecord);

          if a.IPAddress = MailIPAdress then
          begin
             Result := True;
            idDNSResolver.Free;
            Exit;
          end;
         end;
       end;
     end;

  finally
     idDNSResolver.Free;
  end;
end;