[Exim] Freezing on transport child SIGTERM

I'm having a bit of trouble at the moment with the following feature
of Exim: when a delivery process terminates with anything except a
zero exit status, the relevant message is frozen.

However, my system wants to shut down Exim when it starts doing the
backups, to make the filesystem quiescent. This involves sending a
SIGTERM to every Exim process.

So, I end up with frozen messages. What I'm doing at the moment is
manually thawing them, and then they seem to get delivered (or not as
the case may be).

Two questions:

1. Is it safe to send an Exim delivery process a SIGTERM ? If not
then there's probably something wrong, because (for example) on many
UN*X installations memory overcommitment may cause any process to die
at any point with SIGKILL for no particular reason. If it's not safe
then I should go through the code and try to make it safe, I
suppose, and in the meantime it would be nice to have a documented way
to stop an in-progress delivery attempt.

2. If it is safe, would it be a good idea to patch deliver.c not to
freeze messages under these conditions ?

Ian.