System design of an Outlook-style email service: delivery, sync and search

In a technical round for a product manager role at Microsoft, you are asked to design Outlook. The interviewer makes clear that the emphasis is system design: how the product works behind the screen, not only which features it should offer. Treat Outlook as an email service plus the apps people use to send, receive, organize and search their mail. Outlook also carries a calendar and contacts, so agree with the interviewer how much of those belongs in scope, then walk through the architecture you would propose and the trade-offs you would make. ```hint Follow one message Trace a single email from the moment the sender presses send until it appears on the recipient's phone and laptop. Every hop it passes through is a component that needs a design decision. ``` ### Clarifying Questions - Which clients are in scope: web, desktop, mobile, or all three showing the same mailbox at once? - Are we designing the mail service itself (storing mail and exchanging it with other providers), or only a client app that connects to existing mail servers? - Should calendar, contacts and tasks be part of the core design, or treated as services added later? - Is the target consumer accounts, business customers with retention and compliance needs, or both? ### What a Strong Answer Covers - A scoped list of functional requirements (send, receive, sync across devices, search, folders, attachments) and the non-functional ones that matter most for mail: durability, delivery reliability, consistency across devices, latency and offline use. - A component architecture: clients, an API layer, mailbox storage, outbound and inbound mail transfer, sync and notifications, search indexing and attachment storage, with rough sizing to justify it. - A data model and sync approach that keeps several devices consistent, including offline edits and conflicting actions. - Spam, phishing and security handling, plus how the design scales, replicates and fails over. - Product judgment on which technical trade-offs users actually feel, and how you would measure that the system is working. ### Follow-up Questions - A user deletes a message on their phone while their laptop is offline, and later moves the same message to a folder on the laptop. What should happen when the laptop reconnects? - How would you make search across years of mail fast without slowing down the delivery of every incoming message? - What would you change in the design to serve users on slow or metered mobile connections?