Microsoft and SPF
Many deliverability folks stopped recommending publishing SPF records for the 5322.from address to get delivery to Microsoft. I even remember Microsoft saying they were stopping doing SenderID style checking. A discussion on the emailgeeks slack channel has me rethinking that.
It started out with one participant asking if other folks were seeing delivery improvement at MS if they added a SPF record for the 5322.from. Other folks chimed in and said yes, they had seen the same thing. Then I started digging and discovered that MS is still recommending SenderID records on their troubleshooting page.
Email sent to Outlook.com users should include Sender ID authentication. While, other forms of authentication are available, Microsoft currently only validates inbound mail via SPF and Sender ID authentication.
The support page may be out of date, or it may not. In any case, it may be worth adding a SPF record to your 5322.from domain if you’re seeing persistent problems at Microsoft and no where else.
This isn’t great practice overall. But, it may explain why some folks are having such a hard time cracking the MS inbox.