virtual_localuser:
driver = aliasfile
transport = local_delivery
domains = "lsearch;/opt/exim/etc/maildomains"
file = /opt/exim/etc/passwd.$domain
user = ${local_part}
group = mail
search_type = lsearch
modemask = 002
no_more
Like this, I had per Domain an /opt/exim/etc/aliases.$domain and /opt/exim/etc/passwd.$domain. So, for every domain , all local mailaddresses were kept resective aliases and passwd files.