There's been ample evidence of this, many different blogs, notification emails etc.

This thread duplicates the first observations of this which showed up here yesterday at https://news.ycombinator.com/item?id=50404161

My mail provider, MXroute, sent me a notification of the issue early yesterday. There is commentary from various DMARC providers (Suped and PowerDMARC). This is a real thing.

It does not matter which host you are using, this has nothing to do with the hosting provider. It has everything to do with having your domain correctly set up on DNS so that anybody receiving mail will know how to authenticate it. Microsoft has apparently advanced a policy that if you don't even try you don't get to send.

IMO this is the right call, I've seen plenty of spoofing attacks where the attacker took advantage of nonexistent policy on non-sending domains. You might think that if you don't have an MX record the domain can't be used to send mail, that is wrong. The MX record only tells others where to send your mail, it says nothing about whether the domain can send mail itself.