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 large amount of spam messages during this week.
Probably, your computer was infected by a recent virus and now runs a trojan proxy server.

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

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