Exim and virtual mail hosting

[Toby Batch <toby@io.com>]

Does anyone have any good howtos on this?  I've been following the docs 
from exim but I've created my local domain dbm and added it to the 
local_domains directive (list of domains in /etc/local-domains; 
exim_dbmbuild /etc/local-domains /etc/local-domains.dnm)

and added this to my exim.conf:

virtual:
  driver = aliasfile
  domains = dbm;/etc/local-domains
  no_more
  file = /etc/mail/$domain
  search_type = lsearch

I created a (empty) file for each local domain in /etc/mail but when it 
try to send any mail i get:

Exim configuration error
  auth virtual: cannot find auth driver "aliasfile" in line 482

I presume that this is because the aliasfile property in the above 
secification should either a directive or a file name.

What should the alias file directive look like?
What format should the files in /etc/mail for each domain be in?
  e.g. how do i have john@foo.bar deluver to local user john_site99

I've been wrestling with this MTA problem and it's sending me made!