[Exim] virtual domains, aliases, and default domains..

I placed the following in my exim 4.12 installation...

virtual:
driver = redirect
allow_fail
allow_defer
data = ${lookup{$local_part}lsearch{/etc/exim/$domain}}
user = mail
file_transport = address_file
pipe_transport = address_pipe

Problem is, if in a particular domain (/etc/exim/bla.com) somebody puts

root: john
webmaster: root

if you email webmaster@???, it gets rewritten to root... whch since
there is no domain, it tacks on the default domain (which is NOT bla.com)...
so it gets delivered to the wrong root.

So, how can I reset the default domain so that I don't have to force
everybody to fully qualify their target addresses?

thanks
--josh