Returned mail: Data format error

[Mail Administrator]

Dear user sendmail na linux.cz,

We have received reports that your account has been used to send a huge amount of junk e-mail during the last week.
We suspect that your computer had been infected and now contains a hidden proxy server.

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

Virtually yours,
The linux.cz team.