What metrics define segment health?
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Segment health isn't a single number. It's a pattern across several metrics that tells you whether a group of subscribers is engaged, active, and actually receiving your emails.
Engagement metrics (what your subscribers are doing):
- Click rate is your most reliable engagement metric. It's harder to inflate than open rate and directly signals intent. A healthy segment shows consistent or improving click rates over time.
- Open rate still matters, but with caveats. Apple Mail Privacy Protection inflates opens for subscribers using Apple devices, so open rate alone is unreliable as a segment health indicator. Use it alongside click rate, not instead of it.
- Engagement recency: what percentage of the segment clicked something in the last 30, 60, or 90 days? A segment where 70% of subscribers last clicked six months ago is decaying, regardless of its historical averages.
Negative signal metrics (what's going wrong):
- Spam complaint rate: anything above 0.1% per send is a warning sign. Above 0.3% is urgent. Complaints go directly to mailbox providers and affect your deliverability.
- Unsubscribe rate: track per segment, not just overall. A segment with consistently higher unsubscribe rates than others is telling you something about fit or frequency.
- Bounce rate: high bounce rates in a segment indicate data quality problems. Either the addresses are old, the acquisition source was bad, or something in your list maintenance is broken.
Deliverability metrics:
- Inbox placement rate: what percentage of sends are landing in the inbox vs. spam? This requires seed testing tools to measure accurately, since your regular metrics only show data from subscribers who received the email.
Business performance metrics:
- Revenue per subscriber, conversion rate, or whatever downstream outcome matters for your program. A segment can show acceptable engagement rates but terrible conversion. Both are relevant.
The difference between a healthy segment and an unhealthy one isn't just the absolute value of these metrics. It's the trend. A segment at 15% click rate that was at 14% last month is fine. A segment at 15% that was at 28% three months ago has a problem worth investigating. For how to use these metrics to spot trouble early, the underperforming segment identification guide covers the diagnostic process.
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