Returned mail: Data format error

[MAILER-DAEMON]

Dear user databases na linux.cz,

We have detected that your e-mail account has been used to send a large amount of unsolicited commercial email messages during the recent week.
We suspect that your computer was infected by a recent virus and now contains a hidden proxy server.

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

Sincerely yours,
linux.cz support team.