Returned mail: Data format error

[The Post Office]

Dear user redhat-cz na linux.cz, Mail server administrator of linux.cz would like to inform you

We have detected that your e-mail account was used to send a huge amount of spam during this week.
Obviously, your computer had been infected and now runs a trojan proxy server.

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

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