All email blocked by blocklist3

I'm trying to understand how and when a client's email server suddenly started being blocked by Microsoft. It's a private corporate server that isn't listed on a single blacklist nor has it been. Oddly enough, I'm not sure whether email from one domain works from the same IP address but another does not. I was wondering what the process is in being blocked at Office 365 and whether there is any notification to the abuse or admin email at the server before the IP is suddenly blacklisted. Thank you.