Customers still have problems occasionally _sending_ email to one domain, which is over 15 years old and sends <1 email per day. If they initiate and email us we can't send them a reply (if they're on MS email, sometimes). We use an outlook.com email nowadays as a relay and have to treat MS using customers differently still despite using a relatively large supplier.
Some years ago, I was lead to believe, you could pay a third-party to add you to what was effectively MS's whitelist.
Aside: back then I was doing some webdev and supporting IE5+ so I already hated MS about as much as one could.
Never paying to enable interoperability that is part of being a reasonable web citizen/company. Paying them just reinforces the negative behaviour.
There are plenty of other email providers which are worth considering, and I'm sure some of them have half-decent customer support.
You would, sadly, be wrong.
Microsoft customer support is "least bad" among the email providers.
That is a massive indictment of email providers, but it is what it is.
This. So much this.
I will happily run my "artisinal" mail system for myself. Would I put customers on it? Oh, hell, no.
I, sadly, always recommend that companies pay money to Microsoft for email. You are really paying for the customer support service rather than the email service.