Whats the best way to write a letter to a villager in animal crossing gamecube?

I can't figure out the best way to send a proper letter to a villager on animal crossing Gamecube. I've tried the key with the code, I've tried sending a blank mail, I've just tried sending hi, i've tried sending how are you?? and each time I get a nasty gram back. lol. what is the best way?? Im trying to send them fruit so that they can send me foreign fruit back. any suggestions?? thanks!!