I get plenty of custom domain name spam that comes through outlook.com (I'm not even talking about M365/O365). Free or not, Outlook.com is abused for spam constantly.
Fair enough. For $7.00 you can add up to 900 domains to one account and spam away for a month. As long as you setup SPF, DMARC, and DKIM then your messages are likely to be delivered. Switch accounts and domains as often as you like. It is not much of a cost and almost all of it can be automated.
