https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/use-the-delist-portal-to-remove-yourself-from-the-office-365-blocked-senders-lis?view=o365-worldwide it has a link to: https://sender.office.com/ "Office 365 Anti-Spam IP Delist Portal" It's a 3-step process (send verification / confirm email address / Delist IP )