Send emails to ONE recipient in Plain Tx

Hi in exchange 2000 we want the emails sent to one recipient in the domain to be in plain text only. ie Whenever the recipient is either included in a group or sent ot individually the format must be in plain text. We cannot rely on all the outlook users, macintosh users and internet mail users to first select the plain text format before sending to this specific client. How can I achieve this?
Regards.