A notification will be sent to the email account set here if one of the organization's accounts is flagged for sending spam.

A normal user that suddenly starts sending out spam is often a sign that the account was breached and is now being abused. Even if the account was not breached, a spamming account should be temporarily disabled for investigation. This feature is part of the Microsoft 365 Anti-Spam Policies for outbound emails.

You may define multiple addresses to get notified if there is any signal of outbound spam happening within the organization, Office Protect will update the default Anti-Spam policy accordingly. 

You can find the setting in the Security Center, in the Threat Management/Policy/Anti-Spam section.

The operation to look for in the Unified Audit Logs: Set-HostedOutboundSpamFilterPolicy

Microsoft’s documentation about outbound Anti-Spam policy: https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/configure-the-outbound-spam-policy?view=o365-worldwide