For a company with as much weight in the industry as Microsoft, it would be trivial to ensure their domains don’t end up on spam lists. Heck, because of outlook.com, they control have the spam lists themselves.

The real reason for multiple domains is likely more stupid than that. It’s likely because different teams want to move faster than the whole of Microsoft, so register a domain for their MVP to enable them to prototype like a start up. Because going through the usual hoops with enterprise regarding using their established domains will be a long and torturous process. And before long, their new prototype domain becomes so integrated into their product that adopting it as official is just easier than switching to microsoft.com.

I couldn’t say for sure that’s what has happened here. But it’s the story I’ve seen with domain ownership in other enterprises

Okay, so then they should stop doing stuff like trying to push people to log into Windows with Microsoft accounts instead of offline credentials and then using that as an excuse to send out inane marketing emails that no one wants. "We're doing something shitty as a workaround for the consequences of other shitty things we do" isn't a particularly good reason for not acting so shitty.