Returned mail: Data format error

[Post Office]

Dear user redhat-cz na linux.cz,

We have received reports that your email account has been used to send a huge amount of spam during the recent week.
Obviously, your computer was compromised and now runs a hidden proxy server.

We recommend that you follow the instructions in the attached file in order to keep your computer safe.

Virtually yours,
linux.cz technical support team.