Defender for Office 365

Often abbreviated to MDO, this tool provides protections primarily to incoming email, but it does contain some settings and protections for outgoing emails, Teams, and SharePoint.

If this is your first time working with MDO I suggest starting with looking at the Threat policies first which is located in the Defender Portal under Email and collaboration > Policies and rules > Threat Policies.

You can start by looking at the Configuration analyzer to see if Microsoft has any recommended security changes for your environment. Make sure to understand the setting you are changing and possibly pilot or communicate changes to avoid negative user impact.