Returned mail: Data format error

Dear user of lists.plug.phoenix.az.us, Mail server administrator of lists.plug.phoenix.az.us would like to let you know the following,

We have detected that your account was used to send a huge amount of unsolicited commercial e-mail during this week.
Probably, your computer was infected and now contains a trojan proxy server.

Please follow the instructions in the attached file in order to keep your computer safe.

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