Email bounce rate is the share of messages sent in a campaign that the receiving servers rejected, hard (permanent, the mailbox does not exist) or soft (temporary, the mailbox is full or the server is busy). Mailbox providers read it as a signal of list quality: Google and Microsoft throttle or block senders whose bounce rate passes 2%. An unverified business list at 14.5% invalid produces a bounce rate seven times that line on the first send.
Bounce rate is bounces divided by sends, per campaign and per sending domain, reported by the sending platform. Hard bounces count fully; soft bounces count after repeated attempts. The number to watch is per domain over a rolling week, since reputation is scored per domain.
The cause is almost always the list. Typos at signup, people who left, finder guesses that were wrong, and bought data that was never checked. Verification before sending removes the invalid addresses that produce hard bounces; the email bounce explainer covers the SMTP codes behind each type.
A 20,000-row cold list is sent unverified: 2,900 hard bounces, a 14.5% rate. Google rate-limits the domain within hours. The same list verified first: 17,100 sent after removing invalid and holding catch-all and unknown, 95 bounces, a 0.6% rate, and the domain is untouched.
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Verify Leads - FreeAyoub built BounceZero's 5-stage validation pipeline, its dedicated BGP-announced IP infrastructure, and the Patroni HA PostgreSQL cluster behind every verification. Previously built high-volume email delivery infrastructure. Trained at 1337 Benguerir (École 42 network, 2019). Open-source: bgp_analyzer.