Your bounce handler is probably suppressing the wrong address
Here is a bug I have now seen in four separate codebases, including one I wrote. Mail goes out. A bounce comes back. The handler parses the status code, sees a 5.x.x , concludes the address is bad, and adds the recipient to the suppression list. Clean, obvious, and wrong for an entire class of bounce. Because not every 5.x.x is about the recipient. Some of them are about you . Two families, one…
Many developers have encountered a peculiar issue where bounce handlers incorrectly suppress email addresses. When a bounce message with a 5.x.x status code is received, the handler assumes the recipient's address is invalid and adds it to a suppression list. However, this assumption is often incorrect for a significant portion of 5.x.x bounces. Two primary reasons for these mistakes exist:
1. 550 5.1.1 and 550 5.7.1 codes indicate permanent issues. When a recipient's mailbox does not exist (550 5.1.1) or the sender policy rejects the message (550 5.7.1), it is appropriate to suppress the address. However, incorrectly suppressing on 550 5.7.1 leads to unnecessary address removal.
2. 5.7.x codes signify policy, authentication, or reputation issues. These are about the sender, not the recipient. Suppressing them leads to permanent suppression of valid addresses. Each bounce message should be analyzed based on (sending identity, receiving provider) pair rather than the recipient alone. Maintain a separate record of policy refusals, including the sending domain or IP, receiving provider, code, and raw text.
Evaluate aggregates instead of individual events: consistently high 5.7.x refusals from a provider suggest stopping sending to that provider from that domain and resolving the cause. Specific provider codes, like Google's 5.7.26 (authentication failure), can be fixed promptly in DNS. The practical approach is an automatic hold when policy refusals from a provider reach a threshold, temporarily pausing sends from that provider while allowing untouched recipient addresses.
Properly parsing enhanced status codes is crucial. Enhanced codes are not always in the expected location within the response, and some servers may use different codes in the text versus the status field. Regular expressions can help identify enhanced codes, and if none are found, label it as "unknown" rather than assuming a bad recipient.
This bug's absence from initial inspection leads to suppressed addresses being recorded as permanent and never surfacing the actual problem — authentication failure, reputation issues, or provider decisions about the domain. To distinguish between authentication and reputation problems, checking SPF, DKIM, and DMARC against live DNS takes only thirty seconds.
Authentication issues are less severe than reputation concerns, which require DNS changes but have a higher impact.
Written by urgent.news from Dev.to's reporting — not their text. Machine-written — may contain errors; check the original before relying on it.
