[Exim] verify failure hangs SMTP dialogue ?

Greetings,

I'm trying to develop a configuration which supports a number of virtual domains
for our web-based email system. I have exim 3.20.

receiver_verify is on

Everything seems to be ok for valid localparts but I seem to be having a problem when
the receiver _doesn't_ verify ...

I see a line like this in the main log

verify failed for SMTP recipient nosuchuser@??? from <me@???> H=localhost (there) [127.0.0.1] U=whoever

but it appears that the SMTP client doesn't get a 550 respose from exim ...
(I telnet to smtp port to check) and the SMTP session just hangs until it times out.

Can any kind person explain why this happens and how I stop it - or direct me to the appropriate place in the Fine Manual.

I should say that I'm using an aliasfile director which looks like this
# This is supposed to match the virtual users
virtual_user:
   driver = aliasfile
   file = /path/to/per/domain/user/dbms/${extract{2}{.}{$domain}}
   transport = virtual_user_delivery
   search_type=dbmnz
   user=nobody                                                                        f

Thanks in advance ...



--
Jonathan Kyme
jrk@???