[exim] configure accounts for particular domains

hello,

my server is running debian/testing with exim4 and it hosts several
domains.
the default configuration of exim4 supports only global account and
alias settings. what i mean is that an alias configured in /etc/aliases
is valid for all local domains, and the configured storage method for
local system users too.
mail to local_user@??? and to local_user@??? both reach
the same user mailbox.

i already configured a router which enables domain-based aliases, i put
it behind the system_aliases router.
this should have the effect, that when no global alias entry in
/etc/aliases exists, exim4 looks for an entry in the domain-specific
alias file at /etc/exim4/aliases/domain.

the router looks like the following:
virtual_users:
debug_print = "R: virtual_users for $local_part@$domain"
driver = redirect
domains = +local_domains
data = ${lookup{$local_part}lsearch*@{/etc/exim4/aliases/$domain}}
qualify_preserve_domain
no_more

to split the mail for real mailboxes, i could add system users with the
name 'user_domain.com', and then add "user: user_domain.com" to the
domain-based alias file /etc/exim4/aliases/domain.com.

this is a solution, but i guess there is a more comfortable and more
effective one.

what happens if i create a local system user with the username
'user@???'. does he only reach mail to user@??? through
exim4, or whould his mailaddress be user@???@domain.com then?

i hope you can give me some suggestions, as i'm not sure whether my
solution is very suggestive.

bye
jonas