[exim-dev] Debugging SMTP transport crash

I have a message in the queue where an SMTP delivery attempt results
in a segmentation fault:

2019-09-27 21:00:04 recipient@??? R=dnslookup T=remote_smtp_ext defer (-1): smtp transport process returned non-zero status 0x000b: terminated by signal 11

How do I debug this? Is there a way to run the delivery under the
right user using GDB, without forking?

Alternatively, I guess I could capture a coredump somehow, but that
might prove difficult as well due to the use of setuid etc.