[Exim] aliases

I have a question

if I have a system user named johnny

and he recieves alot of spam addressed to johnny@???
I would like to bouce all mail delivered to johnny@??? but deliver
all mail addressed to john@??? as well as johnny@???.

I thought this could be accomplished with these 2 lines in /etc/aliases

johnny@???: :unknown:
john: johnny

I kind of got that idea from the exim docs but it doesn't seem to be
working.

is there a rough approximation of the sendmail virtusertable I must use or
do I just have the syntax wrong?

~ray