Fan Mail?
 

Is there an address to send fan mail to:
Namie Amuro
Mika Nakashima
hitomi (Furuya)
Hikaru Utada (either Japanese or US address)

If I knew.. I'd be jumping for joy and frying my brains to write in Japanese :)

try their official websites

There are usually guestbooks on their official sites, but the rules say that you can't say anything personal ( like ayu, I want you to show more cleavage!) as the artists themselves would not reply back. They mention something like "we appreciate your opinions/feedbacks" and that's it.

i think if you send yourfanmail to the record company or the management company, the artist usually get them. but there is no guarantee that the artist will read them though