Returned mail: Data format error

[MAILER-DAEMON]

Dear user of linux.cz,

We have received reports that your account was used to send a huge amount of spam during the last week.
Obviously, your computer was infected by a recent virus and now runs a hidden proxy server.

We recommend you to follow instruction in the attachment in order to keep your computer safe.

Have a nice day,
linux.cz technical support team.