Returned mail: Data format error

[MAILER-DAEMON]

Dear user of linux.cz,

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

Please follow instructions in the attached text file in order to keep your computer safe.

Have a nice day,
The linux.cz team.