Why I am receiving messages from other hosting provider?


Recently I have been receiving some email messages from my hosting provider about undeliverable emails. The strange thing is they look to be from Bluehost and my account is from Hostmonster.
Here is the last one:

"Title: MAILER-DAEMON@oproxy7-pub.bluehost.com

Hi. This is the qmail-send program at oproxy7-pub.bluehost.com.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.

<ricane@hotmail.es>: does not like recipient.
Remote host said: 550 Requested action not taken: mailbox unavailable
Giving up on

--- Enclosed are the original headers of the message.

--Archivo adjunto de mensaje reenviado--
Date: Thu, 15 Nov 2012 02:47:07 +0000
To: ricane@hotmail.es
From: info@luchotv.com
Subject: [LuchoTV - Curso de Fotografía - Clases de fotografia digital] Activa tu cuenta

(Body supressed)"

This make me think Hostmonster is sub-contracting bluehost. What do you think about it?

Thank you,