We looked at 500,000 bounce logs. Here's what we learned.

We analyzed 500,000 bounce events across 50 client databases to understand what actually causes B2B emails to bounce. The patterns are different from what most email guides tell you.

About 62% of hard bounces were caused by the address no longer existing — the person left the company or the company changed its email format. This is the dominant failure mode and it is almost entirely preventable with regular list verification.

Domain-level issues accounted for 18% of bounces: expired domains, misconfigured mail servers, and corporate email gateways blocking unknown senders. These are harder to prevent because they are not about the individual contact — they are about the receiving infrastructure. But they tend to cluster. If you see 10+ bounces from the same domain in a single send, the domain is the problem, not the addresses.

Another finding: generic role-based addresses like info@, sales@, and support@ had bounce rates 3x higher than individual professional addresses. This surprised us — we expected role-based addresses to be more stable since they are not tied to a specific person. But generic addresses are often unmonitored, abandoned, or protected by aggressive spam filters.

The most actionable insight: bounces cluster by data source. We found that lists purchased from aggregators had 2-3x the bounce rate of lists built through direct verification. Lists scraped from web sources were the worst, with bounce rates exceeding 15% in some cases. If you are buying from a provider who can't tell you how they sourced each record, expect higher bounces.

The good news: most bounces are predictable and preventable. Regular verification, domain-level monitoring, and source-quality scoring would catch the majority of bounces before they happen. The teams that implement these practices see bounce rates below 2%. The teams that don't see bounce rates above 5% and then wonder why their open rates are dropping.