Managing it in small and large corporate enviornments for over a decade led me to believe that when it was working, Exchange was very stable and reliable.. But when things would break, they would break in a way that required extensive knowledge of all the areas to fix. Which was a form of job security so Im not complaining.
I guess past reliability might be irrelevent here if an AI slop patch for AI discovered exploits caused the outage. Just speculation though. Since exchange has so many moving parts and so much legacy code, with even more bolted on to make it a cloud service, I could see how an AI exploit fix would not have all the context needed.
There is no information that it was caused by Exchange either, guess we have to wait on those answers. It is just a reasonable assumption.
Open source options tend to require a bunch of knowledge to set up but after that you generally are equipped with that knowledge to solve any configration issue. I wrote "configuration issue" because aside from "ran out of disk space" none of them really "broke", so it was never "fixing the thing", just the usual mail server admin stuff of figuring out why this or that server didn't like us.
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