There are several security options that can be configured so that you can manage and secure your SharePoint online environment. The following sections describe the controls that are available.
Managing security options
Configuring restricted domains
When configuring options for external sharing, you can choose to provide a list of domains as a security boundary. The list you provide can either be used in an including fashion (only allow sharing to recipients in these domains) or in an excluding fashion (allow sharing to recipients anywhere except these domains). This option is configured via the SharePoint Admin Center | Sharing | Advanced settings for external sharing option, as shown in the following screenshot:
