The official Pokémon movie website has updated. Translations of the new entry follows:

The Singer of the Theme Song to Pokémon the movie XY&Z: Volcanion and the Ingenious Magearna is YUKI!

The Singer of the theme song to the movie "Pokémon the movie XY&Z: Volcanion and the Ingenious Magearna", which premieres Saturday July 16th, will be YUKI!
The theme song will be "Throw Your Voice in the Mailbox (Post ni koe wo nageirete)", which YUKI wrote specifically for this movie.
We have received a comment from YUKI, who has been chosen to sing this theme song!

After it was agreed I'd do the theme song, I read the script to the movie, then wrote the song lyrics.
This is a great honor. I am extremely grateful.

I really love letters.
Because of that, whenever I throw a letter in a mailbox, I always think of the recipient's face.
And that face never fails to make me smile.
Holding other people dear is the same as loving yourself.
I wrote these lyrics thinking about how this was surely how Volcanion felt.
I will send this song to all the "I"s and "you"s out there.
May it ride the wind until it reaches you.


YUKI

A trailer that incorporates YUKI's "Throw Your Voice in the Mailbox" will soon be shown on the "Oha Suta" episode airing on the TV Tokyo Network Thursday April 14th at 7:05am! It will also be shown in theaters all over Japan starting Saturday April 16th! Look forward to it!

Source: http://www.pokemon-movie.jp/news/?p=2027