How can you group bounces by domain or ISP?
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Grouping bounces by domain or ISP turns a single bounce rate number into diagnostic signal. Here's what that analysis actually looks like and what it tells you.
Domain extraction: from each bounced address, pull the part after the @ symbol. That's your domain. Aggregate the bounce counts by domain, then look at which domains are generating the most failures.
ISP grouping: multiple domains often belong to the same provider. Map them:
- Gmail: gmail.com, googlemail.com
- Microsoft: outlook.com, hotmail.com, live.com, msn.com
- Yahoo: yahoo.com, ymail.com, and regional Yahoo domains (yahoo.co.uk, yahoo.com.au, etc.)
What the breakdown tells you: if your bounce rate is elevated at Gmail but normal at Outlook and Yahoo, that's a Gmail-specific reputation issue, not a list quality problem. If bounces are evenly distributed across all ISPs, the source is more likely bad addresses in your list rather than a provider-specific problem.
Most ESPs let you export bounce data with the recipient address. A simple pivot table grouped by domain is usually enough for the analysis. For ongoing monitoring, some platforms (including most modern ESPs) provide per-domain bounce breakdowns in their reporting dashboards. Check your ESP's analytics section before building anything custom. Understanding hard vs. soft bounce classification matters here too, since you'll want to look at both types separately when doing domain-level analysis.
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