What is a message-level event stream?
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When you send an email, it generates a sequence of events: sent, delivered, opened, clicked, bounced, complained, unsubscribed. Most ESPs roll this data up into aggregate campaign stats. A message-level event stream is the raw version: every event, timestamped, tied to a specific message and a specific recipient.
Instead of seeing "open rate: 28%", you see a log entry for each open: who opened it, on what device, at what time. Instead of a campaign bounce total, you see each bounce with the recipient, the SMTP response code, and the exact timestamp.
Why this matters
Aggregate reporting hides problems that per-event data surfaces. If your campaign shows 2% complaints overall, that's useful. But the event stream might show that 80% of those complaints came from a single list segment, or from addresses acquired through one specific source, or from recipients who received more than one campaign in 24 hours. You can't see that in summary stats.
Message-level streams are also what you need for debugging. If a specific address is bouncing with a 550 code, the event log tells you the exact reason and when it happened. Aggregate dashboards often don't surface the raw SMTP responses that tell you why mail is failing.
How to access event data
Most mid-tier and enterprise ESPs expose event streams through webhooks (push) or through their API (pull). Some, like SendGrid and Postmark, make event logs a first-class feature. Others bury it or don't offer raw event access at all. If you need event-level data and your ESP doesn't expose it, that's worth factoring into your platform decision.
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