I manage a service that sends about 25k emails daily using SES (AWS) with some dedicated IPs. Despite everything done right wrt spf/dkim/dmarc, IP reputation warm up etc, we still run into these issues from time to time with multiple providers. Outlook.com just says 'it is being delivered' and then just deletes the email or something, some local ISPs do that too. And then deny it when you try to 'debug' the issue.
This is simply how email delivery 'works' anno 2022 unfrotunatly