As I understand it, message 1RjkC8-0007ue-15 provoked a bounce, which is
fine, but there's no other trace of 1RjkC8-0007ue-15 in the mainlog.
How is this possible? If I send invalid mail via exim4 -i ..., that
doesn't appear in the mainlog either, but the R field isn't present in
the generated bounce: