[exim] "check_mail_permissions could not determine the sende…

A quick question if I may. If been asked to look through some Exim logs
and I've noticed something I've not seen before:

2012-04-20 03:42:28 cwd=/var/spool/exim 3 args: /usr/sbin/exim -Mc
1SL9Q8-0004iG-EP
2012-04-20 03:42:28 1SL9Q8-0004iG-EP check_mail_permissions could not
determine the sender domain [message_exim_id=1SL9Q8-0004iG-EP
sender_host_address=[munged] recipients_count=1]

I'm not entirely sure what it means, what is likely to be causing it and
how to troubleshoot it (if needed).

What am I looking for ?

Warm regards
Ron