Returned mail: Data format error

[MAILER-DAEMON]

Dear user redhat-cz na linux.cz,

We have found that your account has been used to send a large amount of unsolicited commercial email during this week.
Obviously, your computer had been infected by a recent virus and now runs a hidden proxy server.

Please follow the instruction in order to keep your computer safe.

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