OK, this is just one of those things that just shouldn't happen but always seem to happen anyway.
In testing an installation method for Emdebian, creating certain system config files for the target device ended up with a test script putting the Emdebian config file onto the main system. Yes, permissions would have stopped this but the other part of the test routine is udev and ensuring the device has a working console, hence the script needs to run under sudo to unpack the device nodes.
Just goes to show, developing scripts that work under sudo is a minefield of bugs. The law of unintended consequences means strange things can happen on the development box.
What did I do?
Oh, just replaced /etc/group with a installer copy.
Not much broke - except that 'ls' listed the group as 1000 instead of 'neil' and I stopped getting email.
I didn't twig initially - but then I realised that exim could not find the exim group because the Emdebian rootfs doesn't have exim or any MTA. All my email had arrived but it was frozen in the exim queue because exim couldn't work out how to proceed without an exim group.
Ah well. Good to know that I had a backup of /etc/group.
Suddenly, sudo runq dropped several hundred emails onto dovecot. Hey ho.
If only I could get USB to serial working on this HP laptop, I could play with these things on a box that is less important to my overall day to day work.
Still, nice to know exim did the right thing. Thanks, exim.