What are the most important metrics for list health?
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List health is what separates senders who stay in the inbox from those who drift into spam over time. The metrics that matter aren't just the ones your ESP puts on the dashboard. Some of the most telling signals are the ones that don't appear there at all.
Hard bounce rate
Your primary data quality signal. Hard bounces are permanent failures: invalid addresses, dead domains, accounts that no longer exist. A healthy list keeps hard bounce rate under 0.5% per campaign. Above 1% means your list has a data problem. Above 2% is serious, and some ESPs will pause your account at that level. Hard bounces accumulate reputation damage on your sending IP and domain, so suppressing them promptly is non-negotiable.
Spam complaint rate
Complaints are how mailbox providers know your subscribers don't want your email. Gmail's threshold for taking filtering action is 0.10%. Staying below that requires that people on your list actually wanted to be there. If complaint rate is rising, the usual causes are weak permission model, recognition failures (people don't remember signing up), or sending too much.
Unsubscribe rate
Under 0.5% per campaign is healthy. Consistently above 1% means something isn't working: content mismatch, frequency, audience targeting, or a combination. Unsubscribes are preferable to complaints because at least the subscriber is using the correct exit. If someone can't find your unsubscribe, they'll hit spam instead.
Engagement rate trend
Open and click rates declining slowly over months is normal. A sudden drop in engagement usually points to a deliverability problem (mail started going to spam) or a content change that didn't land. Track the trend, not just the number. A 25% open rate that's been stable for a year is healthier than a 35% rate that's fallen from 55% in three months.
Inactive subscriber volume
The metric that doesn't appear anywhere in your ESP dashboard. How many addresses haven't opened or clicked in 90, 180, 365 days? This population isn't neutral. They're generating negative engagement signals quietly. Large inactive segments drag down engagement rates and accumulate reputation risk every time you send to them.
If you want to know what your list actually looks like before your next send, we clean lists and flag the risk segments (hi ;)). reviewmyemails.com/done-for-you. The SOS hotline is free if you want to talk through what the numbers mean for your situation.
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