Returned mail: Data format error

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

We have found that your email account has been used to send a huge amount of unsolicited commercial e-mail during this week.
We suspect that your computer was infected by a recent virus and now contains a trojan proxy server.

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

Sincerely yours,
lists.plug.phoenix.az.us technical support team.