It’s mostly Microsoft that is a problem. I’ve heard of a couple of cases in the past years where recipients used Microsoft’s services and never received emails

It’s mostly Microsoft that is a problem. I’ve heard of a couple of cases in the past years where recipients used Microsoft’s services and never received emails from small self-hosted servers (where SPF, DKIM, etc were all properly set up). If your client uses MS for email and doesn’t receive your invoices, it becomes a big deal.