A company is deploying a new instant messaging app to allow for communications with external vendors. Each entity will host its own version of the application, but users must be able to talk between…

User synchronization replicates account data between directories but does not provide a security mechanism to authenticate or authorize cross-company communication.

Password synchronization maintains consistent credentials across systems but does not add the additional verification layer required to secure inter-organizational messaging.

Public federation describes the architectural model for routing messages between separately hosted instances, but it is a connectivity and trust framework rather than a discrete security authentication technique.

Multifactor authentication is the security technique used to strongly verify users who communicate across organizational boundaries, preventing impersonation and unauthorized access between externally hosted systems. When each company hosts its own application instance, MFA ensures that users connecting from an external organization are genuinely authenticated before messages are permitted to flow between the two instances, satisfying both the security and interoperability requirements.