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Bounce Codes Explained: What Every Sender MUST Know to Protect Deliverability (Expert Guide)
Published: 12/4/2025
A detailed breakdown of SMTP bounce codes, what they mean, how mailbox providers interpret them, and how Impressionwise helps you prevent them. Bounce codes are one of the most important — and most misunderstood — deliverability signals. They tell you why emails fail, but they also tell mailbox providers how responsibly you manage your list. Unfortunately, most senders misread bounce codes, ignore patterns, or rely on outdated assumptions. As a result, they miss the early warnings that mailbox providers use to flag risky senders. This guide breaks down bounce codes clearly, simply, and in a modern context — showing you how to interpret them properly and how predictive hygiene tools like Impressionwise help prevent them. What Are SMTP Bounce Codes?Bounce codes are numeric status messages sent by a receiving mail server (e.g., Gmail, Microsoft, Yahoo) that explain why an email cannot be delivered. They follow the SMTP RFC standards, but each provider has its own interpretations and proprietary versions. A bounce code tells you:
Bounce codes = the receiving server’s opinion of your mail. Hard Bounces vs Soft BouncesUnderstanding the difference is essential: Hard Bounce (Permanent Failure). The email will not deliver. Ever. Reasons include:
Hard bounces = remove immediately. Soft Bounce (Temporary Failure). The email might deliver later. Reasons include:
Soft bounces = monitor, retry, or suppress intelligently. Why Bounce Codes Matter More TodayFrom 2023–2025, mailbox providers changed how they evaluate senders. Bounce behavior is now one of the top three signals used to determine:
Mailbox providers expect clean lists and safe behavior. High bounce rates are interpreted as: “This sender acquires emails irresponsibly.” You cannot “outsend” poor hygiene anymore. The Major Code Families (2xx, 4xx, 5xx)Every bounce code is part of one of these families: 2xx — Success
Example: These are the only good codes. Anything outside 2xx indicates a rejection or warning. 4xx — Temporary Failure Examples:
4xx signals mean: “Your email might be OK, but we’re not ready to accept it.” They can also indicate reputation-related throttling. 5xx — Permanent Failure Examples:
A 5xx means: “Never send to this address again.” 5xx errors should trigger suppression. The Most Common Bounce Codes & What They Mean (Modern Interpretation)Here are the codes senders encounter most often: 550 — Mailbox Unavailable (MOST COMMON HARD BOUNCE) Meaning:
Remove immediately. 551 / 553 — Not Our User / Invalid Mailbox Meaning:
Remove immediately. 554 — Message Rejected / Transaction Failed Meaning:
This is rarely about the address — it’s about your reputation. 421 — Temporary Server Issue Meaning:
Retry later. 450 — Mailbox Unavailable Possible causes:
Monitor; may be reputation-related. 451 — Local Error / Temporary Processing Error Often triggered by:
If repeated, it’s a reputation warning. 552 — Storage Limit Exceeded (Mailbox Full) Mailbox full = user likely inactive or abandoned. This is an early signal of:
Monitor these addresses carefully or sunset. Accept-All Domains: Why They Confuse Verification
Accept-all (catch-all) domains always respond:
“250 OK” — even if the user doesn’t exist. This makes verification unreliable. These domains:
Many verification-only tools incorrectly mark these as “valid.” Impressionwise identifies them as high-risk profiles based on domain behavior and activity patterns. Hidden Bounces (Blocklist & Filtering Failures)Not all bounces return clear SMTP codes. Some mailbox providers silently:
...without sending traditional bounce responses. Symptoms include:
This is why reputation monitoring + risk scoring is essential. How Bounce Patterns Impact ReputationMailbox providers look at:
A high or erratic bounce pattern signals:
This leads to:
Keeping bounce rates below 1% is now a best practice. How Impressionwise Prevents Bounce IssuesThis is where Impressionwise outperforms simple verification vendors. Impressionwise uses three synchronized layers: 1. Predictive Verification Unlike basic SMTP checks, Impressionwise evaluates:
This results in more accurate deliverability prediction. 2. Behavioral Risk Scoring Addresses that look valid can still bounce later. Impressionwise identifies:
This reduces future bounces, not just present ones. 3. Source Integrity Mapping Bounce clusters often originate from:
Impressionwise traces these patterns, enabling:
Together: Impressionwise reduces:
This stabilizes inbox placement, boosts deliverability, and strengthens long-term sender reputation. Final RecommendationsTo protect deliverability and sender reputation: DO
✔ Keep hard bounces < 1% DON’T
✘ Assume “valid” from verification = safe With predictive analysis, Impressionwise helps you detect and eliminate bounce risk before it hits your sender reputation. |
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