alias with no primary domain

[M.A. GEERTSMA]

I configured my server with no/fake (primary) domain. So I only serve 4 virtual 
domains.But then the /etc/alias file is of no use for these domains. Is it true 
that I have to use the /etc/postfix/virtual for definining all possible email 
adresses? Or is there a way I can use a alias file for virtual domains 
too?Thanks