Your question is Slack MVP under Time Constraint. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Design Slack, with a constraint of delivering an MVP within 2 weeks; discuss message-delivery scalability and reliability.
Asked in the system_design stage as a one-hour main-loop system design interview. The reported follow-up focused on how to achieve message-delivery scalability and reliability.
Address:
Discuss the tradeoffs between direct fanout and brokered delivery, push versus pull synchronization, and strong versus eventual consistency. If proposing ML, explain why it belongs in the MVP and how it would be isolated from the critical delivery path.