Go to https://mail-tester.com. Send a test email. If you don't get 10/10, fix your shit.
Also why they do non-standard stuff like accepting mail but then simply not delivering it, rather than bouncing it.
"Reputation scores" and blacklists / greylists started in late 1990s / early 2000s as multiple grassroots efforts when spam started to overwhelm systems. Gmail did not exist, outlook.com did not exist, it was easy and expected to run your own SMTP (which I did). And it was very unpleasant.
DMARC appeared 10 if not 15 years ago. There's no reason for a sane MTA configuration not to have it.
- it was a way to outsource the mail server administration and already free in our corporate package
Which is pretty much all I have from the corporate side. On the personal or small business side its most often
- I don't really care that much about running my own stuff
- I needed a personal email address
All these are valid reasons/motivations but none explicitly argue that email messaging should become proprietary.
DMARC applies not on your mail server, inbound or outbound, but on the server that is receiving your mail (which may in fact be your mail server).
That server will look at how the mail authenticates and decide if it should allow the mail to proceed, it (for the most part) will then tell you whether or not it allowed the mail. This is how you will know you might have a deliverability problem before your customers do.
If you're wanting to know about deliverability (particularly for this Microsoft case), my company (Alesian Security) has a free check at https://alesiansecurity.com/try?tab=deliverability
It's been a while, but monitoring DMARC seemed pointless once you got things started. If my outbound mail fails to validate, I've asked the receivers to bounce it, and I should be able to see that on my own system.
Otherwise, I'm just getting reports about spoofed spam or people who need to adjust their mail forwarding, but I can't do anything about either.
But it provides a safe guard against accidentally broken mail flows as your email stack evolves over time which is where it pays dividends.
I would love to say that it would have alerted you ahead of time to this particular event, but given that it's the fix here you would never have seen it.
Some can affordably deliver to Microsoft because they aggressively police what they are actually sending on their customers' behalf; others don't get involved and you end up having to spend a fortune on a dedicated IP.
The DMARC thing is just another part of this; one of my clients discovered this silent failure themselves a few weeks back and addressed this.