[exim] single file lookup

I'm playing around with a single-file lookup

======
file: domain.com
----------------------
a    : /usr/local/mail/domain.com/a
b    : a@???
----------------------

from what i remember in exim 4.(<=3) , mail addressed to b@???
should be redirected to a@???, -- without me doing anything
magical correct?

i keep getting this in the log:
defer (-21): appendfile: file or directory name "@domain.com/Maildir"
is not absolute

I had something like this working on a test machine a few years ago,
but I can't remember what i did

my router is:

{{{
virtual_domains:
   driver        = accept
   transport     = virtual_domains__maildir_delivery
   domains       = +local_domains
   condition     =  
${lookup{$local_part}lsearch{/usr/local/etc/exim/virtual_domains/ 
$domain}}
}}}

i thought about chaining a redirect router before this, but i swear i
did this once before in 1 router.

anyone have a clue?