[Exim] Config question

I have a problem that I cannot solve maybe one here can help me.

I have set up exim to use smarthost on my linux (debian) box. Incoming mail is handled properly by fetchmail, but with outgoing I must map the username here to another username on the smarthost site.

e.g. username foo@domain should be mapped to bar@domain
how can I do this ?

Thanks in advance
  Rainer
-- 
Dr. Rainer Gehring                  Tel: +49 7247 824187
Forschungszentrum Karlsruhe         Fax: +49 7247 822849
Institut für Technische Physik
Rainer.Gehring@???