Open a ticket with Office 365. If you are sending mail through them and getting blocked, that means your messages are being sent out from the 365 High Risk Pool:
https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/high-risk-delivery-pool-for-outbound-messages?view=o365-worldwide#high-risk-delivery-pool
The high risk delivery pool is a separate IP address pool for outbound email that's only used to send "low quality" messages (for example, spam and backscatter). Using the high risk delivery pool helps prevent the normal IP address pool for outbound email from sending spam. The normal IP address pool for outbound email maintains the reputation sending "high quality" messages, which reduces the likelihood that these IP address will appear on IP block lists.
The very real possibility that IP addresses in the high-risk delivery pool will be placed on IP block lists remains, but this is by design. Delivery to the intended recipients isn't guaranteed, because many email organizations won't accept messages from the high risk delivery pool.