Are you suggesting the three people should be required to use a mailserver?
I can't speak for Jacques, but when I deliver software, I try to avoid any breaking changes for as long as possible. In the event that breaking changes are absolutely unavoidable--and there seems to be a question about whether that applies in this case--then I communicate clearly beforehand in simple terms about WHY the change is necessary, and WHAT the steps are to implement it, and I ensure buy-in on the part of those using my code. I try to make such changes as simple as possible, preferably "run this script and check the simple expected output."
That does not seem to be the focus here. The update might not have been strictly necessary in the first place, and if it was, it wasn't communicated consistently well to customers, and the communication doesn't seem to have been so simple and straightforward, either. Barring the communication, humane error messages would be nice, but those are also lacking.
I guess I would add to your list: when a company hires sysadmins, security engineers, and network engineers, they should also hire some UI people and good product managers to communicate with the outside world.