How do ISPs detect poor hygiene?
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Mailbox providers are not naive. They have been fighting spam for decades and have sophisticated systems for identifying senders with poor list practices. Here is what they actually watch.
Bounce patterns: Every hard bounce is logged. If your sends consistently return high bounce rates on a domain or IP, providers notice. A bounce rate above 2% on most platforms starts raising flags. A sudden spike tells them something changed in your list or acquisition process.
Complaint rates: When subscribers hit "this is spam," many providers log it and some send it back to you via feedback loops. Google Postmaster Tools shows you Gmail's spam button press rate. Keep this below 0.10%. Anything above 0.30% will trigger filtering.
Spam trap hits: Anti-abuse organizations like Spamhaus seed email lists with trap addresses. When those traps receive mail, they report the sender. Providers use these blocklist signals as part of their filtering decisions.
Engagement patterns: If a large fraction of your recipients never open, never click, and occasionally mark as spam, providers weight that in their filtering decisions. Engagement signals are increasingly central to inbox placement, especially at Gmail.
The tools providers give you to see their signals: Google Postmaster Tools for Gmail-specific reputation data, Microsoft SNDS for Outlook, and feedback loops for Yahoo and Outlook complaint data. Check these regularly. Our free Blocklist Checker is a quick first check for your domain and sending IP.
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