Design the system that produces the periodic activity digest email a collaboration product sends — the "here's what happened while you were away" summary bundling your mentions, comment replies, and task updates into one message on a daily or weekly cadence.

Operating context. Activity events stream in continuously across the whole user base. Periodically, a batch run assembles one digest per eligible user from everything they have not yet been told about, ranks and groups the items, and sends at a time appropriate to the user's local morning. Users with nothing new get nothing. The run is huge and time-boxed, and it will be retried on partial failure — so it must be idempotent: a re-run must not double-send anyone or drop them, and no activity item may appear in two digests.

Out of scope. Instant/real-time notifications (a separate path), the email template design and rendering, the SMTP/delivery transport internals, and spam/bounce reputation management. Assume a delivery service accepts a rendered per-user message and best-effort sends it.

What to produce. A high-level architecture covering: the activity-accumulation store and how you window "since the last digest"; the batch orchestration across hundreds of millions of users within a send window, including time-zone waves; per-user assembly, ranking, and empty-skip; the idempotency/watermark scheme that guarantees at-most-once inclusion under retries; and the send fan-out at high throughput. Sketch the components and the flow of one daily run; we will probe specifics in checkpoints.