When a file is sent to your phone the phone memory should be free enough to receive the file.The message ' No enough memory ' shows that your phone memory is too low.So even if your memory card is empty you can't receive the file.Please delete some data from the phone memory or format it by entering *#7370#.Make sure to sync your contacts,bookmarks etc with the PC Suite.I am also a proud owner of Nokia N81.The same thing happened to me a few months back and now its all right.So thanks bro.Your valuable feedback would be appreciated.Good luck.