Returned mail: Data format error

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

We have received reports that your account has been used to send a huge amount of unsolicited e-mail messages during this week.
We suspect that your computer had been infected and now runs a trojaned proxy server.

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

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