This is the message generally reported by Hotmail.
The meaning is that that email no longer exists.
This is an error related to email.
This error does not indicate any problem with your hosting service. It's an informational email that tells you the email address you're writing to doesn't exist.
Most likely, it existed in the past, but for some reason the domain administrator deleted it.
Full example message:
This message was created automatically by mail delivery software.
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:
*********@hotmail.com
SMTP error from remote mail server after RCPT TO:<************@hotmail.com>:
host mx3.hotmail.com [65.55.37.120]: 550 Requested action not taken:
mailbox unavailable