Uncategorized Email Bounce Codes and What They Reveal About Your Reputation By MailChannels | 3 minute read When emails fail to deliver, they bounce back with strange numeric codes like 550 5.7.1 or 421 4.7.0. These are SMTP bounce codes, and they’re much more than error messages—they’re a window into your IP and domain reputation. In this guide, we’ll break down: What bounce codes are and how to interpret them Which codes signal reputation problems How mailbox providers use bounces to assess your trustworthiness What to do if you’re seeing high bounce rates What Are Email Bounce Codes? Bounce codes are status messages returned by receiving mail servers when they reject or delay delivery of an email. There are two main types: Hard bounces (5xx codes): Permanent failure. The email will never be delivered. Soft bounces (4xx codes): Temporary failure. The server may retry delivery later. Example: vbnet CopyEdit 550 5.7.1 Message rejected due to sender’s IP reputation This means your email was rejected outright—likely because your IP is flagged as untrustworthy. Bounce Codes That Signal Reputation Problems Here are the most common bounce codes that indicate IP or domain reputation issues: CodeMeaningWhat It Reveals550 5.7.1Blocked or rejected messageYour IP/domain has poor reputation554 5.7.1Message not acceptedSpam filtering blocked you421 4.7.0Deferred temporarilyVolume too high or reputation too low550 5.7.0Policy rejectionAuthentication failure or blacklisting571Delivery not authorizedPossibly an open relay or lack of SPF/DKIM550 5.4.1Relay access deniedMisconfigured relay or flagged IP pool Mailbox providers don’t always tell you outright when they’re blocking you for reputation. These codes are often your only clue. Why Bounce Codes Matter for IP & Domain Reputation Mailbox providers use bounces to: Score your sending behavior Throttle delivery if you send too fast or too much Detect compromised accounts or spam bursts Adjust spam filter rules based on your track record Repeated bounce codes like 550 5.7.1 tell providers you’re not a trustworthy sender—even if your emails are legitimate. How to Monitor and Interpret Bounces Enable SMTP logging on your mail server Use a delivery monitoring platform like Postmaster Tools, SNDS, or your relay provider Analyze bounce patterns: Are they happening at one domain (e.g., Gmail)? Are they increasing over time? Are they tied to a specific sender or type of email? If your bounce rate is consistently above 5%, you’re at risk of being throttled, filtered, or blacklisted. How to Fix Bounce-Related Reputation Issues Improve List Hygiene Remove invalid or bouncing addresses Stop emailing unengaged users Use double opt-in to avoid spam traps Authenticate Your Email Set up SPF, DKIM, and DMARC correctly Align envelope and header domains Monitor for Compromised Accounts Watch for sudden spikes in outbound volume Disable scripts or plugins sending abusive mail Use a Smart SMTP Relay Relay services like MailChannels detect reputation-related bounces and act on them before your IP gets blocklisted. How MailChannels Helps MailChannels SMTP Relay includes: Real-time bounce handling and diagnostics IP reputation protection and pool isolation Abuse detection to stop problematic senders ResponseAnalytics: detailed insight into why emails bounce Instead of guessing what 550 5.7.1 means, you get actionable insight—fast. Try MailChannels for Free → Summary: Bounce Codes to Watch Bounce CodeReputation Signal550 5.7.1Likely blacklist or IP block554 5.7.1Spam content or bad domain421 4.7.0Temporary throttle (volume/reputation)571Open relay or missing authentication