Upon alert from Microsoft Defender for Office 365 about suspicious user activity, what is a recommended immediate action?

The suggested immediate action of turning off delegation for the user's account is crucial in a scenario where suspicious user activity has been detected. This step helps to limit the potentially malicious actions that the user could perform, especially if their account has been compromised. Delegation allows users to have access to another user's mailbox or to send emails on their behalf, which could be used to further exploit the situation or spread potential threats. By disabling delegation, you instantly reduce the risk of unauthorized access and actions, thereby controlling the situation more effectively while further investigations can be conducted.

This action aligns with best practices in incident response, where immediate containment measures are taken to mitigate the potential impact of suspicious activity. It allows security teams to investigate the suspicious behavior without the threat of further misuse of the account. Implementing additional measures, such as blocking URLs or scanning emails, might be relevant later but should not be the first step taken until delegation is adequately addressed.