Okay, instead of taking this at face value (with a big acknowledgment that it is genuinely frustrating that both Google and Microsoft don't have good contacts to just know what went wrong), I'll analyse this carefully.

This website is operated by Virtual Industries Group (https://www.vig.co.nz/), which is according to their website operates three services: School Bookings/School Interviews (https://www.schoolinterviews.co.nz/, focused on school scheduling and the service discussed here), Care Bookings (https://www.carebookings.co.nz/, which is the same service as above but focuses on day-care and other similar functions), and MessageMyWay (https://www.messagemyway.com/, which according to their website "is the communications hub for your community. It is your emergency communications plan, your telephone tree, and your email broadcast system all in one").

From a cursory glance, these three services shares this set of outbound MX servers. While it is very unlikely that someone who uses School Interview and Care Bookings would mark their message as spam, if the messages relayed by MessageMyWay is sent on the same outbound MXes then I could immediately see the problem. A large part of MX operators knows this and separates "marketing" and "operational" messages into separate servers to prevent this exact thing from happening. While I understand this dev's frustration, maybe the messages relayed by MessageMyWay are the ones marked as spam by frustrated parents who are receiving irrelevant school marketing which are sent to same MXes as their important operational messages?

If I were the developer (and still insist on using on-prem email), I'll operate three groups of servers:

Set A: purely for company-initiated messages, never for the customers

Set B: "operational" customer-initiated messages: School Bookings, Care Bookings, and MessageMyWay mails which are marked "critical" by the users (which is apparently additional cost)

Set C: "marketing" customer-initiated messages: "normal" MessageMyWay mails