That with a side of suppressing a competitor. Similar to how they include Teams for corporate plans. If it is included in your M$ apps suite, then your company might want to cut back on Slack and just make due.

How do you send email tho? Its my understanding that hosting is easy, but major providers will assume you are spam and block? Or do you just receive?

I have been de-googling for a while and proton was next on my list