Returns engagement counts (opens and clicks) grouped by the email client, operating system, or device type they were recorded from, for the requested period. Use it for the classic "opens by mail client" view -- for example the share of opens from Apple Mail versus Gmail versus Outlook. The reading environment comes from open and click events only, so this breakdown reports engagement activity, not delivery: it has no delivery counts and no open/click rates, because delivery is reported by the receiving mail server without a client or device and there is therefore no per-client delivered denominator. Use `group_by` to choose the facet: `email_client` (default), `os`, or `device_type`. Each row populates the chosen facet and leaves the other two null. Rows are ranked by the `sort` metric (default `unique_opens`) descending and capped at the requested `limit` (default 50, hard maximum 200). Rows are computed against event time (not send time). The maximum window is 365 days; requesting a longer range returns 422. Start date (inclusive) in YYYY-MM-DD, interpreted as a calendar day in `timezone` (a UTC day when `timezone` is omitted). Defaults to 30 days before `to` when omitted. End date (inclusive) in YYYY-MM-DD, interpreted as a calendar day in `timezone` (a UTC day when `timezone` is omitted). Defaults to today in that timezone when omitted. Window may not exceed 365 days. IANA timezone identifier (for example `Asia/Kathmandu` or `America/New_York`) to report the statistics in. It is the single source of timezone: day and hour boundaries, and the relative window defaults used when `from` and `to` are omitted, are computed in this timezone instead of UTC, so a timezone with a sub-hour offset (such as India at +05:30 or Nepal at +05:45) still gets correct local-day and local-hour totals. When it is set, a `from` or `to` given as a calendar day names a local day in this timezone, and one given as an instant stays an absolute point in time but is rounded down to its hour and bucketed in this timezone (so the hour boundaries are local, not UTC). To avoid specifying the zone twice, a `from` or `to` that carries its own numeric UTC offset (for example `+05:45`) is rejected when `timezone` is set; use a calendar day or a `Z` (UTC) instant instead. Defaults to UTC. Email statistics are aggregated across all categories. Per-category filtering is not yet available; supplying this parameter returns 422. Which reading-environment facet to group rows by. `email_client` (default) groups by mail client; `os` groups by operating system; `device_type` groups by device type. Each row populates the chosen facet and leaves the other two null. Metric to rank rows by, applied descending. Defaults to `unique_opens`. Only engagement counts are sortable; this breakdown has no rates. Maximum number of client rows to return, ranked by the `sort` field descending. Inclusive start date the response covers (YYYY-MM-DD). Inclusive end date the response covers (YYYY-MM-DD). The instant the statistics in this response are current to: events recorded up to roughly this time are reflected, while more recent events may not be yet. Statistics are served from a rolling aggregation that refreshes every few seconds, so a response is near-real-time but not live; use this field to label data freshness (for example "as of 14:03") rather than assuming the numbers are to-the-second. Null when the freshness boundary is not being reported. Client breakdown rows, ranked by the `sort` metric (default `unique_opens`) descending. Empty when no opens or clicks with a detected client occurred in the period. The mail client this row aggregates (for example `Gmail`, `Apple Mail`, `Outlook`). Populated only when `group_by=email_client`; null otherwise. The operating system this row aggregates (for example `iOS`, `Android`, `Windows`, `macOS`). Populated only when `group_by=os`; null otherwise. The device type this row aggregates (for example `mobile`, `desktop`, `tablet`). Populated only when `group_by=device_type`; null otherwise. Distinct open events, counting repeat opens from the same recipient and opens auto-fetched by inbox privacy features (such as Apple Mail Privacy Protection and the Gmail image proxy). Distinct open events excluding those auto-fetched by inbox privacy features. Same event-counting semantics as `opens`, with prefetched opens removed. Distinct recipients who opened at least once, including opens auto-fetched by inbox privacy features. Distinct recipients who opened at least once, excluding opens auto-fetched by inbox privacy features. Distinct click events, counting repeat clicks from the same recipient. Distinct recipients who clicked at least once. Total number of distinct values of the requested `group_by` facet with activity in the period, regardless of `limit`. When it exceeds the number of rows returned, the ranking was capped; raise `limit` (up to 200) or narrow the window to see more.