[Exim] logging :defer:ed transactions

I have the following in my configuration:

------------------
virtual_aliases:
        domains = lsearch;/etc/exim/domain
        driver = aliasfile
        file_transport = address_file
        forbid_pipe
        file = /etc/exim/aliases/$domain
        search_type = lsearch
        user = nobody
------------------

and the following in /etc/exim/domain/example.com

------------------
bob: :defer: a good reason why I can't accept mail right now
------------------

During an attempt to deliver to bob@??? a 451 code is returned:
451 <bob@???> a good reason why I can't accept mail right now

However there is no record of this transaction in exim's log.

How can I enable logging of these transactions?

--
Lee Maguire <lee@???>