A few days ago I stopped getting email notifications from my automatic droplet snapshot scripts. I could clearly see the backups were being done on the DigitalOcean end, but no notifications.
I decided to run one from the console this morning and all became clear. The script checks if the server is back up and running after a power down and snapshot by pinging the droplet.
I'd introduced firewalls on them a few days ago (coincidentally on the same day I stopped getting notifications) and hadn't allowed pings. Leaving a console open while there was a loop going on awaiting a ping, I allowed pings on the firewall and straight away the script continued and ended properly.
Mystery solved. :-)