Yes you can.

User signs up, uses @outlook. Your app declines it and explains that they block with no way to remedy, and explain they are a bad vendor.

Deeply unserious take.

	▲	nekusar 2 hours ago \| parent [-]
		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.