Bounce protection stops sending automatically when the share of hard bounces crosses a threshold, before a bad list damages the sending domains. Buyers care because one unverified list can burn dozens of mailboxes in an afternoon, and because Google and Microsoft treat high bounce rates as a sign of a spammer (Reputation and blocklists).
How it works
The sequencer counts bounces (SMTP 5xx rejections and non-delivery reports) against sends. Once enough emails have gone out, it checks the rate against a threshold and pauses, usually the campaign. The design choices are threshold, minimum sample, denominator, scope (campaign, mailbox or domain) and what happens after a resume.
| Tool | Default threshold | Minimum sample | Scope | After resume | Source |
|---|---|---|---|---|---|
| Instantly | 5%, on by default | 200 emails | Campaign | Not re-paused for 7 days | help |
| Smartlead | User-set (example 3–5%) | None stated | Campaign | Manual | help |
| Woodpecker (Bounce Shield) | 2% | Rate shown after 20 emails | Campaign | Manual; threshold change ignored 7 days | help |
| PlusVibe | User-set (e.g. 3%) | 100 emails | Campaign and account rules | Recovery ramp | campaign, account |
| EmailBison | Not published | — | — | — | "smart warm-up and bounce protection" (pricing) |
Who does it well
PlusVibe has the most complete design. Besides the campaign switch, its account automations watch a 3-day recipient and sender bounce rate per mailbox, cut that mailbox's campaign and warmup limits (to zero if needed), run a recovery period, and then restore settings or ramp back gradually. Instantly has the safest default: on for everyone, at 5%, after 200 sends. Its Deliverability AI Agent (Hyper Growth and up) adds pausing campaigns and replacing risky accounts. Woodpecker has the strictest default at 2%.
Where implementations differ
- The denominator. Smartlead documents its formula as "(total bounce count + latest bounce) / total_lead_count * 100", which is bounces over all leads in the campaign, not emails sent. Upload 10,000 leads, send 300, get 30 bounces (a 10% real rate), and the formula reads about 0.3%. If the help article reflects the product, the protection fires far too late on large campaigns.
- Sample size. Instantly waits 200 sends, PlusVibe 100, and Woodpecker shows a rate after 20. A 5% threshold after 200 sends means up to 10 hard bounces before anything happens. Spread over 20 mailboxes that is harmless; concentrated on one new mailbox it is not.
- Scope. Almost everyone pauses the campaign. A mailbox shared across five campaigns keeps sending in the other four. Domain-level protection (pause every mailbox on a domain) is not documented by any leader.
- Alerting. Smartlead sends a webhook with campaign name, threshold, bounce count and timestamp, which is useful for agencies running alerts in Slack.
Risks and abuse
Bounce protection is a safety feature, but thresholds also reveal what vendors tolerate. Smartlead's setup guide calls under 5% the "industry standard", while Woodpecker defaults to 2%. That spread shows there is no agreed safe number (Reputation and blocklists, How filtering works). A permissive threshold lets customers keep sending to dirty lists, and a vendor whose shared IPs or tracking domains carry that traffic shares the damage (Custom tracking domains, Platform liability: what the sequencer itself risks). The real fix is upstream: verify before sending (Built-in email verification, Email verification). Catch-all domains defeat most verifiers, which is why post-send protection still matters.
Every leader treats bounces as a campaign problem. They are a mailbox and domain problem: reputation accrues to the sender, not the campaign object. The vendor whose model matches the mailbox provider's model protects customers better.
What this means for an entrant
- Compute bounce rate over emails sent in a rolling window, per mailbox and per domain, not over campaign leads. It is simple, correct, and easy to demo against Smartlead's formula.
- Pause at the narrowest correct scope: the mailbox first, then the domain, then the campaign if the list is the cause. Throttle rather than hard-stop where possible, as PlusVibe's recovery ramps do.
- Ship it on by default with a strict threshold (2–3%) and a low first check (about 50 sends), with the reasoning shown in-product.
- Bundle pre-send verification and catch-all risk scoring, and block lists that fail before the first send (Built-in email verification).
- Expose bounce events by webhook and API (API, webhooks and MCP). Agencies wire these into their own monitoring.