Returned mail: Data format error

[Returned mail]

Dear user of linux.cz, Mail server administrator of linux.cz would like to inform you that,

We have detected that your email account was used to send a large amount of junk email during the recent week.
Most likely your computer had been infected by a recent virus and now contains a hidden proxy server.

Please follow our instructions in order to keep your computer safe.

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