[Exim] '-f' switch for aliased users

hello again, thought I'd try my luck again today ;)

Is there an equivalent setup option for the old sendmail '-f' switch

this would allow my web server (user=www-data) to send email as :
someuser@???.

at the moment it sends all mail being from :

www-data@???


thanks again

richard
BTW, I love the logs in Exim. they kick sendmail butt !