Unfortunately those kind of decisions tend to be left up to C-level individuals who generally only look at the costs associated with that decision.
Hell, even MIT has succumbed to this pressure - they are in the process of migrating the entire campus from an onsite hosted Microsoft exchange server system to cloud hosted microsoft 365 email system. To the laments of the users and the IT staff who have to support it.
From a system that has served a massive user group like MIT successfully with little downtime for over a decade to a system that has already caused multiple issues - even when they are still migrating people after 6 months...
Exchange on premises is dead. Out rather dying a slow death in a legacy environment.
Have you tried to set up MFA for an on prem Exchange system? Well, it is simply impossible. MFA on activesync ? Impossible.
When you have to support such a legacy environment you are better off moving to Microsoft 360 (I think this is their new name), or gmail or others similar players.
Not just cost, they are also allergic to things like mailbox quotas and maximum attachment sizes. Everybody wants to shoot 25+MB attachments at each other and store it in their mailboxes and then summon it from a search 5 years later. Microsoft has recognized this customer demand and it's something m365 does pretty well. Your onprem PST gets corrupted if you sneeze at it but microsoft will let you have a 100GB mailbox with 1.5TB archive on their service and you'll never lose anything or suffer a slowdown.
oh my bad. I'll turn the spam filter off for your address. let me know if there's anything else I can do and be sure to rate this interaction 5 stars.
I actually do think this is an ATP issue. I've had a few premier cases for it and the guys manning the desk at microsoft dont know how to deal with it. We always have to get a japanese speaker to raise the case in that langauge because the JP support guys are way better than the english.
The problem is that no decision maker at Microsoft is actually going to promise in writing that even 99.9999% of the email data will be retained, and does not somehow mysteriously vanish, no matter how much money you spend with them. Let alone an actual 100%.
So it's pointless to make such a claim that no one at Microsoft HQ even approximately believes.
In general you should avoid doing your email through giant providers like Microsoft and Google. They just don't care. Smaller providers are more interested in ensuring that everything is actually working.
The small providers can get caught in the gears of large providers or even smaller providers using idiotic RBLs (sometimes a single one that causes a permanent reject). Nobody dares block Gmail or O365 however.
But you won't have an undiagnosable spam filter. Depends on your risks.
We ran our own some time ago and worked closely with the state government who also hosted their own email at the time. They had Barracuda e-mail filters that they used, and they would constantly flag our emails to them as spam at random times. Of course, we could just pay Barracuda a verification fee and get the green light. They couldn't even whitelist us themselves! But those fucking spam filter appliances were everywhere at the time. It sounds like this is just another case of a shitty spam filter.
Hell, even MIT has succumbed to this pressure - they are in the process of migrating the entire campus from an onsite hosted Microsoft exchange server system to cloud hosted microsoft 365 email system. To the laments of the users and the IT staff who have to support it.
From a system that has served a massive user group like MIT successfully with little downtime for over a decade to a system that has already caused multiple issues - even when they are still migrating people after 6 months...