I originally had only my MX hosts listed in my SPF record, and configured to accept them exclusively. Microsoft unilaterally blocked the entire /23 my VPS resides within, and refused to exclude my configured IPs from their block.

The only way I’m currently able to deliver anything to domains hosted by Microsoft is to expand my SPF record to include my ISP’s mail hosts, and route delivery through them.

Microsoft, and the other SMTP cartel thugs, are undermining the protections these protocols were designed to provide.