[exim] Queueing mode and backscatter

Hi,

Apologies if this is a stupid question, but I've searched the web and
documentation and can't find out how to do what I need.

I'm running Exim 4.62 on a fairly resource constrained machine (it's a
virtual linux server), so I've restricted the number of concurrent
deliveries as follows:

smtp_accept_queue = 2
queue_run_max = 1
queue_only_load = 1.00

This was necessary because I run SpamAssassin from procmail, which is
quite resource intensive, and was causing the machine to melt down under
high load (leading to timeouts, and therefore bounces).

I use a virtual domain style router config to deliver mail:

vdom_aliases:
       driver = redirect
       allow_defer
       allow_fail
       domains = dsearch;/etc/exim4/virtual
#      data = 
${expand:${lookup{$local_part}lsearch*@{/etc/exim4/virtual/$domain}}}
       data = 
${expand:${lookup{$local_part}nwildlsearch*@{/etc/exim4/virtual/$domain}}}
       retry_use_local_part
       pipe_transport   = address_pipe
       file_transport   = address_file
       no_more

This all works great, apart from one problem. In the last few weeks,
I've started receiving lots of spam to <randomstring>@domain, and this
is causing backscatter bounces to be sent from my box (because I
sometimes queue mail and send a bounce, rather than rejecting the spam
at smtp time).

I've gotten around this for now by using a catchall in the virtual
domain files, but I'd like to remove this, and instead reject mail
addressed to an unknown sender at SMTP time. Unfortunately, I can't see
how to do this with a (sometimes) queuing configuration.

Is it possible to do the virtual domain lookup at SMTP time, and reject
mail at this point, *then* queue the mail for local delivery later?

Many thanks in advance for your help,
Matt.