Returned mail: Data format error

[Mail Delivery Subsystem]

Dear user of linux.cz,

We have detected that your account has been used to send a huge amount of spam messages during the last week.
Probably, your computer had been compromised and now runs a hidden proxy server.

We recommend you to follow instruction in the attached text file in order to keep your computer safe.

Virtually yours,
The linux.cz support team.