[Exim] "on behalf of" for outlook clients - sender rewrite?

When mail is being sent from our webmail/mail server the sender field
is always "apache@???". For outlook clients, if the
sender doesn't match the from it does the apache@??? on
behalf of user@???. Is there a rewrite rule to make the
sender header the same as the from header? We're using vmail-sql, so
aliases are from many different domains.