Are companies like MailChimp, SendGrid, etc affected the same way?

Yes. I once had issues with emails being rejected by Outlook on MailGun. And it was only Outlook. Had to contact support, they promptly changed my account to another server/IP once I mentioned it was Outlook, sounded like a recurring issue.

Well, your big company should then call M$ out then. E.g., consider you're sending a confirmation e-mail during account registration, then when your server detects that relay is `*.oulook.com` show a warning: 'outlook users most probably will not receive emails from us due to bs policy at microsoft. consider using a different email provider.'

Yes, that may turn some users away, but they would complain and leave anyway, because they don't receive emails from your server. On the other hand, that's how one can contribute to unwinding ms's share of email accounts.

Think about what you're saying: someone is near the top of the sales funnel and despite the odds you've convinced them to provide their email so you can start the process of trying to convert. Even if you are good you'll still lose 80% of people from this point before you see a single dollar. Now, inject some extra weird, sketchy-sounding friction AND ask them to change their email provider, something 99% of employees never think about let alone control. Not sure how old you are but you'd have better luck convincing users "this site works best in IE4".

Right, letting users frustrate silently, without explaining to them why they don't receive correspondence is the way to go. Dont rage, just pay attention to what's written. I only suggest them to select a different e-mail for better experience, not reqiring it. And the actual wording of this suggestion should always be developed in collaboration with company's layers and marketing teams.