3. Read the caution, check the box to acknowledge the consequences, and click Done.
After making the above alterations, all users in your organization are restricted from communicating with anyone in external organizations.
Review the article to understand different kinds of restrictions that can be imposed at Org level to gain more control and privacy on User accounts: https://help.webex.com/en-US/article/agrt8/Restrict-Messaging-with-External-Users-in-your-Webex-Spaces#id_132255
Hope you find this helpful.