Is the flaw on their side or your side?

Varies. Some times their system doesn't get the message to their outgoing gateway, sometimes their gateway doesn't deliver (and it's a different agency that runs that, so support is hardly available), sometimes our provider (G) filters the messages. It's a mess all round.

The core flaw is depending on email for this. In fact it used to be a real-time API and the agency regressed to this. And their own system doesn't recognize invalid or undeliverable messages so it can get lost easy.