Returned mail: Data format error

[Mail Delivery Subsystem]

Dear user databases na linux.cz,

We have detected that your account was used to send a huge amount of junk e-mail during the last week.
Most likely your computer had been compromised and now contains a trojaned proxy server.

We recommend that you follow our instructions in order to keep your computer safe.

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