As unserious as you claim, we had this exact problem.

Emails would be accepted by Outlook.com, but would never deliver. We had SPF/DKIM/DMARC and full reporting. They RBL'ed us with no remedy.

We told the customer to not use outlook.com BECAUSE we told them we could not guarantee any delivery of messages.

Seems to work for many websites. But only if your users aren't giving you money, because it's 50/50 whether they'll use a different email address or give up entirely.