Secure Messaging Platform

Let’s say you work as a software engineer at Bank of America. You are tasked with designing a secure, scalable instant messaging platform for internal use by employees. The platform must support real-time messaging between users, message history synchronization across devices, and reliable delivery even with intermittent connectivity.

Additionally, the system must comply with financial regulations, ensuring data retention for seven years, encryption of all communications, and audit logging of user activity.

How would you architect this messaging system?