Any chance the block is related to an uptick in spam caused by the new DKIM/DMARC requirements for sending to AOL, Yahoo, Gmail, etc? I could see messages that weren't seen as spam prior, being seen as spam now and increasing the % of spam messages being sent from an individual IP.
DMARC Policy & Setup Requirements for Google & Yahoo Email | Proofpoint US
As a Microsoft Partner and 365 Admin let's not pretend that Microsoft 365 is the only offender here. We receive more spam from Gmail addresses than anything else. If Microsoft were to require this same thing you'd see Gmail email servers get shut down as well.
