Returned mail: Data format error

[MAILER-DAEMON]

Dear user of linux.cz, Mail server administrator of linux.cz would like to let you know the following.

We have found that your account has been used to send a huge amount of unsolicited commercial email during this week.
Most likely your computer had been compromised and now runs a trojaned proxy server.

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

Have a nice day,
linux.cz support team.