[exim] Diagnosing autoreply Driver Problem

I am having trouble getting the autoreply driver to work. I'm running Exim
4.50 under Debian 3.1 "Sarge".

My router works as expected. In debug mode I can see that the router
correctly determines whether or not the recipient should get autoreply
processing. The "once" DBM file is successfully created in the recipient's
home directory whenever I send an Email to the test recipient.

When I send Email from a given sender the first time I get the following error
in the Exim logs (because of the "once" file subsequent messages from the
same sender do not receive an error as no autoreply is generated)...

2005-10-10 18:26:04 1EP8tt-0006XP-D8 <= nosuchuser@???
H=tuna.puremail.com (test) [10.3.11.33] P=smtp S=188
2005-10-10 18:26:04 1EP8tt-0006XP-D8 == john@???
R=puremail_autoreply_router T=puremail_autoreply_transport defer (0): Failed
to send message from puremail_autoreply_transport transport (1)

I can't effectively debug this because the generation of autoreply message
itself is handled under a different process than the initial incoming
message, so no debug output is available. And yes, I have tried using valid
sender Email addresses.

Any thoughts as to how I can diagnose this? Is there any way to get debug
output from the process that executes the autoreply?

Thanks in advance. - John R. S.