[exim] Address Rewriting help

Hi,

    I'm trying to rewrite all the from headers coming from a server so 
they all appear from the same user. Exim is only needed for outbound 
mails, not incoming. I have the following line, which mostly works

@beam.tv obscured@??? Ffrs

but emails still show as from whichever user sent them, e.g. "root
<obscured@???>". Is there any way to change the email to appear as
from another user rather than just change the address, e.g. "support
<obscured@???>" no matter which user sent the email?

Adam.