[Exim] problem sending to local users

I am having a problem sending mail to local users using just user_id

ie: mike rather than mike@???

I have this in my config

qualify_domain = bristolreccc.co.uk (seems to just get ignored for
recipients)

The error I get is
user@???
    Unrouteable address

If I add the domain it works, though thats a bit long-winded.

exim version exim-4.32