External access is a Microsoft Teams communication mode that lets people in one organization communicate with users outside the tenant under configured controls. It is different from guest access: external users are not fully added to the workspace, but they can still be reached for chat or calls when the policy allows it.
In cybersecurity, external access matters because it expands the trust boundary around collaboration tools. Attackers can abuse it to contact employees while posing as IT, a partner, or a vendor, making phishing and social engineering feel more legitimate. Defenders use tenant policies, allowlists, restrictions on unknown domains, logging, and user verification procedures to reduce this risk. Review external access settings carefully, because a permitted channel can become an initial-access path if identity is not checked.