Why Are My Emails Bouncing? Complete Bounce Troubleshooting Guide

Bounce messages (Non-Delivery Reports or NDRs) contain SMTP status codes and human-readable explanations. The status code structure: X.Y.Z where X=category (2=success, 4=temporary, 5=permanent), Y=subcategory, Z=specific code.

Always read the full bounce message. Beyond the status code, the text often tells you exactly why the email bounced and what to fix.

550 5.1.1 User unknown: The email address doesn't exist. Remove from your list immediately.

550 5.1.2 Bad destination mailbox address: Invalid domain or malformed address.

550 5.7.1 Message rejected due to policy: Your IP or domain is blacklisted, or the content triggered a spam filter. Check blacklist status and review email content.

550 5.7.26 DMARC policy violation: Your email failed DMARC authentication. Check SPF and DKIM configuration.

554 5.7.1 Service unavailable: Your IP is on the receiving server's block list. Check blacklist status or contact the mail provider directly.

1. 1

   Check if bounce spike is sudden

   Compare bounce rate over last 7 days vs. previous 7 days. A sudden spike (vs. gradual increase) usually indicates a specific event: a new import, a blacklisting, or a configuration change.

2. 2

   Identify the dominant bounce code

   Export bounce log from your ESP. Group by SMTP error code. If 90% are 550 5.7.1, the problem is blacklisting or policy. If 90% are 550 5.1.1, the list has invalid addresses.

3. 3

   Check blacklist status

   Run a comprehensive blacklist check on your sending IP and domain. 550 5.7.1 and 554 5.7.1 bounces at scale often indicate a new blacklisting. Address removal first.

4. 4

   Check authentication records

   550 5.7.26 (DMARC failure) bounces indicate broken SPF or DKIM. Check that your DNS records are valid and that your ESP is signing with the correct key.

5. 5

   Segment recent list additions

   If the spike followed a list import, test a sample of the new addresses. High bounce rates on new segments indicate the source list has poor quality or is outdated.

6. 6

   Implement suppression and monitor recovery

   Remove or suppress all hard-bounced addresses immediately. Monitor bounce rate daily for 2 weeks. If it stays elevated after addressing root causes, escalate to ESP support.

421 4.7.0 Sending too quickly: You're hitting rate limits. Implement throttling — send slower.

450 4.2.1 Mailbox temporarily unavailable: Recipient's mailbox has a temporary issue. Retry after a few hours.

452 4.2.2 Mailbox full: Recipient's inbox is over quota. Retry a few times; if persistent, suppress the address.

421 4.7.28 IP reputation problem: Gmail-specific temporary rejection due to poor IP reputation. Slow down, review list quality, and monitor Postmaster Tools.

451 4.7.1 Greylisting: Server temporarily rejects first delivery attempt from unknown senders. Your MTA should retry automatically after a few minutes.

If your bounce rate suddenly increases:

Recent list import: Check whether the spike correlates with adding new contacts. New segments often contain higher invalid address rates.

Domain changes: If you recently changed your sending domain, the new domain may not have established reputation.

Blacklisting: A blacklisting can cause 550 5.7.1 bounces from many servers simultaneously. Check your blacklist status immediately.

SPF/DKIM/DMARC change: A DNS configuration change may have broken authentication. Check authentication validity.

ESP issue: Sometimes ESPs have infrastructure problems causing widespread temporary failures. Check your ESP's status page.