Need help getting MicroSoft server spam lifted

I have received a notice for undeliverable email and the website contact reported my IP address (assigned to a Microsoft Outlook server) is the reason. They replied, "We have seen a large increase in the amount of phishing spam coming from Outlook servers to our traps and users, resulting in their ratios being above our listing threshold at times. There is nothing I can do to stop or slow the spam from Outlook.   You will have to take your complaint to Microsoft as only they can control the spam volume from their network so the IP will delist."

What the heck? I don't send SPAM and will move my email to another server if that's the answer. Anyone having this issue and have a solution? Would appreciate your help. Thanks.