An open event payload includes: event_type (“email.opened”), contact_id, campaign_id, message_id, opened_at (ISO 8601 timestamp), and ip_address plus a parsed user_agent string for the client that triggered the open. One caveat worth knowing: open tracking relies on a tracking pixel loading, which Apple Mail Privacy Protection and some corporate proxies pre-fetch automatically — so you may see opens recorded that don't reflect an actual human reading the email. Click and reply events are more reliable signals if you need higher-confidence engagement data.