One very mundane reason a company I had worked for switched to Office365, was that emails from our own domain would often end up in the spam filter.
It can cost a lot when that happens.
I see this being a problem in the current situation, where most businesses use either Google or Microsoft for their emails. But in the case of an EU-wide change, I think the situation would be different. Plus, there are other providers that could be used that aren't blocked by MS' and Google's spam filters.
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