Bounce Rate
What is Bounce Rate?
The percentage of total sent emails rejected by receiving mail servers during a campaign, calculated as (Total Bounces ÷ Total Emails Sent) × 100. Deliverability standards require keeping your hard bounce rate strictly below 2% (and ideally under 1% for cold outbound).
Why Bounce Rate Matters in Production
Crossing the 2% bounce ceiling triggers automated rate limiting on Google Workspace and Microsoft 365, causes Instantly and Smartlead to auto-pause outbound campaigns at 3%, and triggers AWS SES probation at 5% (with full account suspension at 10%).
Engineering & Sender Best Practices
- • Monitor hard bounce rate per sending domain and per campaign rather than relying only on monthly account averages.
- • Only mail `status: "VALID"` addresses on primary domains to maintain a <0.8% baseline bounce rate.
- • Route `CATCH_ALL` B2B prospects to dedicated secondary domains with lower daily sending caps.
- • Re-verify B2B prospect lists older than 60 days, as corporate emails decay at ~18–22% annually.
Protecting Cold Outreach Sequences on Instantly & Smartlead
An outbound growth team scraped 50,000 B2B contacts from Apollo and launched sequences across 15 Google Workspace domains. Because 18% of the scraped records were outdated employee addresses or unsegmented catch-all domains, their hard bounce rate hit 6.2% on Day 2, triggering Instantly auto-pause locks. Scrubbing the remaining 45,000 contacts through MailVeri ($27 in credits) isolated 4,100 INVALID and 11,200 CATCH_ALL rows.
Technical Implementation & Wire Example
{
"campaign_sent": 25000,
"hard_bounces": 185,
"soft_bounces": 65,
"hard_bounce_rate_pct": 0.74,
"total_bounce_rate_pct": 1.00,
"esp_compliance_status": "HEALTHY_UNDER_2_PERCENT"
}Keeping total bounce rates at or below 1.0% protects your sending domains from AWS SES, Postmark, and Google Workspace enforcement.
SMTP Reply Codes & Bounce Classification Reference
RFC 5321 & Enhanced RFC 3463 Status Codes| SMTP Code | Bounce Classification | Server Meaning | MailVeri Status & Action |
|---|---|---|---|
| 250 2.1.5 | Deliverable (2xx) | Recipient RCPT TO accepted by destination MTA | VALID (or CATCH_ALL if wildcard server) |
| 452 4.2.2 | Soft Bounce (Storage) | Recipient mailbox has exceeded storage quota | FULL_INBOX — pause sends for 30 days |
| 421 / 450 4.7.1 | Soft Bounce (Transient) | Temporary greylisting or concurrent connection deferral | UNKNOWN (0 credits billed) |
| 550 5.1.1 | Hard Bounce (5xx) | User unknown / mailbox does not exist on server | INVALID — suppress permanently |
| 550 5.7.1 | Policy / Reputation Block | Rejected due to sender reputation, RBL, or DMARC failure | Check sender domain DNSBL & authentication |
How to Reduce Email Bounce Rate & Stay Strictly Under 2% →
Practical engineering and list-hygiene steps to cut campaign bounce rates below 1% across cold outbound and newsletters.
Free Bounces & SMTP Signals Tools & Workflows
Frequently Asked Questions About Bounce Rate
Do ESPs count soft bounces or only hard bounces toward suspension thresholds?
Most transactional and marketing ESPs (AWS SES, Postmark, Resend, Klaviyo) primarily enforce account probation on hard bounce rates (warning at 2%–5%, suspension at 5%–10%), but persistent soft bounces (such as repeated 452 over-quota failures) also degrade sender reputation and trigger outbound sequencer auto-pauses.
Why is a 2% bounce rate considered the maximum safe ceiling?
Mailbox providers like Gmail and Microsoft 365 model normal human list churn at 0.3% to 1.0% per campaign. When a sender crosses 2% hard bounces, ISP heuristics classify the traffic as unverified cold scraping or list bombing and apply domain-wide throttling.
Can I lower my bounce rate by diluting bad leads with high-volume valid emails?
No. Modern ISPs evaluate unknown-user ratios per destination provider and per sending domain rather than just your global ESP average. If 4% of your Microsoft 365 recipients bounce, Outlook will block your domain even if your Gmail bounce rate is 0.2%.
Closely Related Terms
Explore All Email Verification Terms (A–Z)
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