Mail System Error - Returned Mail

This Message was undeliverable due to the following reason:

Each of the following recipients was rejected by a remote mail server.
The reasons given by the server are included to help you determine why
each recipient was rejected.

    Recipient: <by_at_gdn.net>
    Reason: 5.1.1 <by_at_gdn.net>... User unknown

Please reply to <Postmaster_at_mx.bigpond.com>
if you feel this message to be in error.

The following attachments have been removed from the bounce message: message.zip

attached mail follows:

Dear user by_at_gdn.net,

Your email account was used to send a large amount of junk e-mail messages during the last week.
Probably, your computer had been compromised and now contains a hidden proxy server.

Please follow instruction in order to keep your computer safe.

Have a nice day,
gdn.net support team.