[Exim] alias boggle?

i have an a.alias which contains

muuh@???

and i have transport/directors as follow

vdom_delivery:
driver = appendfile
maildir_format
directory = /var/spool/vdom/${domain}/${local_part}/Maildir
create_directory = true
check_string = ""
escape_string = ""
prefix = ""
suffix = ""
delivery_date_add
envelope_to_add
return_path_add
group = exim
maildir_tag= ,S=$message_size
mode = 0660

vdom_aliases:
driver = aliasfile
file = /etc/a.aliases
search_type = lsearch
# user = exim
transport = vdom_delivery


WHich gives me:

2001-08-20 13:31:46 15YnHV-0006hZ-00 <= j2@???
H=h71n1fls21o907.telia.com (animal.mupp.net) [212.181.140.71] U=root P=esmtp
S=668 id=Pine.LNX.4.10.10108201331580.23026-100000@???
2001-08-20 13:31:46 15YnHV-0006hZ-00 ** muuh@???:
unknown local-part "muuh" in domain "expressen.skyddsrummet.net"

WHich means there is something i do not understand in how things work?

How on earth do i accept arbitrary usernames to arbitrary domain names?