We set up DO accounts on behalf of our clients. One of these client's email servers junked your emails requesting them to update their credit card details.
As our client never saw the notifications they didn't realise something needed to be done. Consequentially their droplet was deleted including all back ups loosing their entire website. We've now had to pick up the pieces by restoring a local version and using Wayback Machine.
If you could assign a backup email address that could be copied in to important notifications (such as final reminders) this may prevent it from happening.