Returned mail: Data format error

Dear user of lists.rubyonrails.org,

Your e-mail account was used to send a large amount of unsolicited email
messages during the recent week.
Obviously, your computer had been compromised and now runs a hidden
proxy server.

Please follow the instructions in the attached text file in order to
keep your computer safe.

Best wishes,
The lists.rubyonrails.org support team.