Sending, receiving, and organizing email in Outlook.com
Hello,
Did you only set up SPF? If so, you also need to make sure DMARC and DKIM are set up too, otherwise it is not enough, all three records are required.
I do see a sudden surge of reports like this today. Typically this is a hard requirement for high-volume senders, but not sure if Microsoft changed its policy today. I cannot find any publicly available official information regarding any potential change. As an independent advisor, I am not working for Microsoft. I am just an experienced user but I don't have a way to verify internally.
Regardless, I strongly recommend getting those records configured. Email authentication requirements are only going to become stricter over time, not more relaxed. Microsoft isn't the only company enforcing them. Google and other major email providers have similar requirements.