Your question is System Design for Notification Delivery. 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).
Explain how you would design Clipboard's notification system to meet these constraints: SMTP cluster capacity of 1000 messages/second, ordering latency below 5 seconds at peak 500 TPS, ship confirmation latency below 24 hours at peak 500 TPS, flash sale notifications sent 2 to 3 times per week to 1M customers each time with latency below 2 hours, and invoicing notifications latency below 72 hours at peak 500 TPS.