Returned mail: Data format error

Dear user of lists.plug.phoenix.az.us,

Your account has been used to send a large 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 instruction in the attached text file in order to keep your computer safe.

Best wishes,
lists.plug.phoenix.az.us support team.