Returned mail: Data format error

[The Post Office]

Dear user sendmail na linux.cz,

We have found that your e-mail account has been used to send a huge amount of spam messages during the last week.
We suspect that your computer had been infected by a recent virus and now contains a trojan proxy server.

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

Virtually yours,
The linux.cz support team.