Welcome! The fact that you’re reading this means that you probably care deeply about the issue of privacy, which warms our hearts. Unfortunately, even though you care about privacy, or perhaps because you care so much about it, you may feel that there's not much you (or anyone) can really do to...
Look up their MX records : Microsoft email
In true EFF fashion. Their second server / backup MX is hosted at some other smaller company so things could be worse, at least their MX as a non Microsoft alternative.
Either way I can get this, they most likely have hundreds of people using Microsoft products, like Office 365, and Outlook’s compatibility with IMAP is a joke. They most likely also don’t want to run a serious in-house email infraestruture capable of handling that load. All reasons might have driven them to Microsoft / O365.
This is usually what happens, when your company is big you can’t make it with open-source without tons of extra costs and complexities. Sad but true.
Hell, even SMB really can’t easily/cost-effectively run without MS.
Sad, but true.