Bounce Detection

4 min read

Also known as: Bounce Handling, Bounce Processing, NDR Detection

Automated identification of email bounces (delivery failures) to trigger suppression, sequence exit, and list hygiene actions.

Definition

Bounce detection is the capability of an email or sequence platform to automatically identify when a sent email fails to deliver — either permanently (hard bounce: invalid address, domain not found) or temporarily (soft bounce: mailbox full, server unavailable). Bounce detection parses incoming bounce notifications from receiving mail servers and classifies the failure reason via SMTP response codes.

Detected bounces typically trigger automatic actions: hard bounces add the address to the suppression list immediately, soft bounces are retried for 24-72 hours before reclassification, and persistent bounces (a soft bounce that keeps failing) eventually get treated as hard bounces. The platform also exits bouncing contacts from active sequences to prevent further wasted sends.

Modern bounce detection distinguishes between many bounce types: invalid address (550 5.1.1), domain not found (550 5.4.0), mailbox full (552 5.2.2), blocked by recipient (550 5.7.1), greylisted (450 4.2.1), reputation block (554 5.7.1), and dozens more. Each requires different handling.

Why It Matters

Bounce detection is the first line of defense for sender reputation. Continuing to send to known-bad addresses is the fastest way to damage your domain's standing with mailbox providers. Automatic bounce detection + suppression keeps your list clean and your reputation intact.

The biggest mistake is treating all bounces identically. Hard bounces should suppress immediately. Soft bounces should retry but eventually escalate to suppression if persistent. Reputation-block bounces (where the recipient blocked YOU specifically) require investigation, not blind suppression. Different bounce types need different responses.

Examples in Practice

A SaaS company's outbound sequence sends 5,000 first-touch emails. Bounce detection reports 60 hard bounces (1.2%) and 80 soft bounces (1.6%) within 24 hours. Hard bounces auto-suppress; soft bounces are retried twice over 48 hours, after which 50 of the 80 deliver and 30 are reclassified as hard. Final clean list: 4,910.

An ecommerce brand's transactional email volume suddenly shows a spike in 'recipient blocked you' bounces from one specific domain. Investigation reveals their domain was flagged by that recipient's spam filter after a phishing campaign spoofed their address. They submit removal requests and the bounces resolve within a week.

A marketing-ops team builds custom logic for bounce handling: hard bounces → suppress immediately. Soft bounces → retry 3 times over 48 hours, then suppress. Reputation blocks → flag for manual review and don't suppress until investigated. The nuanced approach keeps legitimate recoverable contacts active while protecting reputation.

Frequently Asked Questions

What is bounce detection?

Automated identification of email delivery failures. The platform parses incoming bounce notifications from receiving mail servers, classifies the failure reason via SMTP response codes, and takes appropriate action (suppression, retry, sequence exit).

What's the difference between hard and soft bounce detection?

Hard bounces (5xx response codes) are permanent failures — invalid address, non-existent domain. Soft bounces (4xx response codes) are temporary — mailbox full, server unavailable. Hard bounces suppress immediately; soft bounces are retried.

How fast should bounces be detected?

Hard bounces typically arrive within minutes of the send and should be processed in real-time. Soft bounces may arrive over hours or days as retries fail. The platform should process bounce notifications within minutes of receipt and update contact status immediately.

What actions should bounce detection trigger?

Hard bounce: add to suppression list, exit any active sequences, log the failure reason. Soft bounce: retry per the platform's retry schedule, escalate to hard if persistent, exit sequences after N consecutive failures. Reputation blocks: flag for investigation, don't auto-suppress.

What's a healthy bounce rate?

Hard bounce rates under 0.5% per send are healthy. 0.5-2% indicates list-quality issues worth investigating. Above 2% indicates serious problems (purchased lists, stale data, signup-form bugs) that damage sender reputation rapidly.

Can bounce detection distinguish bounce reasons?

Yes — modern systems parse SMTP response codes and bounce body text to classify reasons: invalid address, domain not found, mailbox full, message too large, blocked by recipient, reputation block, etc. Each reason can trigger different handling.

What happens if bounce detection breaks?

Hard-bounced addresses continue receiving sends, damaging sender reputation. Suppression list stops growing while bad addresses accumulate. You'll see increasing delivery problems over time without an obvious cause. Audit bounce detection monthly by comparing detected bounces to mail-server logs.

Should bounce detection work for transactional and marketing email separately?

Yes — most senders maintain separate suppression lists for marketing versus transactional. A contact whose marketing emails bounce shouldn't necessarily be suppressed from transactional (password resets, receipts), which they may still need. Keep the suppression scopes separate.

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