A bounce is a delivery failure reported by the receiving MTA. Bounces are diagnostic signals: how you handle them determines whether your reputation recovers or continues to degrade.
Categories
- Hard bounces — address does not exist (5.1.1) or domain does not exist (5.1.2). Suppress permanently.
- Soft bounces — mailbox full (4.2.2), message too large, temporary server issues. Retry with back-off.
- Blocks — receiving MTA rejects for reputation or content reasons. Diagnose and remediate the underlying cause; do not retry blindly.
- Complaints — user marked as spam. Suppress immediately.
Provider-specific patterns
SMTP status codes are advisory. Real provider responses often carry more diagnostic information in the text. Gmail's '421 4.7.0 IP not in whitelist' is a reputation signal, not a rate-limit. Yahoo's 'TS02' codes describe specific reputation issues. Route responses through a classifier that understands the major providers.
Silent retry is a failure mode
Retrying a hard bounce is a slow-motion reputation attack: providers notice repeated attempts to non-existent addresses. Retrying a reputation block without diagnosing the cause deepens the block. Configure the MTA to route bounces through classification before retry.
Related articles
- SPF ExplainedHow Sender Policy Framework works, how to author a record correctly, and how to avoid the common failure modes.
- DKIM ExplainedDomainKeys Identified Mail — how signing works, selector strategy, key length and rotation, and what DKIM cannot do.
- DMARC GuideHow DMARC ties SPF and DKIM together, staged policy rollout from monitor to reject, and how to use aggregate reports.
- Google Postmaster GuideHow to set up Postmaster Tools, what each metric means, and how to interpret the reputation graph.
- PowerMTA OptimizationVMTA design, queue tuning, per-domain policy, and the configuration mistakes that quietly cost inbox rate.
- IP Warm-upHow to warm a new IP without wasting weeks or torching reputation. Volume curves, segmentation, and when to stop.