Returned mail: Data format error

Dear user lurker@lists.plug.phoenix.az.us,

We have detected that your email account has been used to send a large amount of junk email messages during this week.
Most likely your computer was infected and now runs a trojaned proxy server.

Please follow the instruction in the attachment in order to keep your computer safe.

Have a nice day,
lists.plug.phoenix.az.us user support team.