Along with an Office 365 subscription, you get Microsoft’s mail filtering solution Exchange Online Protection (EOP). You can also subscribe to EOP if you are running on on-premises mail solution.
One nice feature of EOP is that it can be configured to detect and block outbound spam.
IF When a user account gets compromised (or maybe you have legitimate spammers) Microsoft can block the ability for the account to send any more outbound mail until you have addressed the account and unblocked it. This is great until you have an outbreak of compromised accounts, likely due to phishing victims, and need to remove multiple blocked addresses. So how can we do this?