Best Practices Diagnosing SMTP 5xx and 4xx Error Codes in Transactional Email By MailChannels | 4 minute read When your transactional emails bounce or fail to send, the first clues often show up in the SMTP response. These are the 3-digit codes, like 550, 421, or 554, returned by mail servers during the SMTP handshake. They tell you why a message was rejected, delayed, or deferred. In this guide, we’ll break down what SMTP 5xx and 4xx error codes mean, how to diagnose them, and how to fix common issues so your transactional emails make it to the inbox. Why SMTP Error Codes Matter Unlike marketing emails that may go unnoticed, transactional emails are time-sensitive, users expect them immediately. When something goes wrong, SMTP error codes are the most direct way to understand what happened and why. These codes are generated during the sending process and logged by your email platform or ESP. By learning how to interpret them, you can reduce failed deliveries, troubleshoot faster, and improve your overall deliverability. Understanding SMTP Error Code Ranges Before diving into specific errors, it helps to understand the difference between 4xx and 5xx codes: 4xx (Temporary Failures): These are soft failures. The message wasn’t accepted right away, but it might still go through after retries. Think of them as temporary delays, often due to greylisting, server throttling, or busy receiving servers. 5xx (Permanent Failures): These are hard failures. The receiving server rejected the message outright. It won’t be retried, and something needs to change before it can be delivered. Common 5xx SMTP Errors and How to Fix Them 550 – Requested action not taken: mailbox unavailable This usually means the recipient email address doesn’t exist, was mistyped, or has been deactivated. Fix: Double-check for typos, use email validation tools, and clean your email lists regularly. 553 – Mailbox name not allowed The recipient’s address is improperly formatted or not accepted by the recipient server. Fix: Ensure the “To” email address follows standard formatting and matches domain rules. 554 – Transaction failed / Message rejected This is a generic hard failure. It may be triggered by spam filters, blocklists, or policy restrictions. Fix: Check your sending domain/IP reputation, scan your message content, and ensure proper SPF/DKIM/DMARC setup. 552 – Exceeded storage allocation The recipient’s mailbox is full and can’t accept more messages. Fix: There’s nothing you can do here, retry later, or ask the recipient to clear space. Common 4xx SMTP Errors and How to Handle Them 421 – Service not available / Connection refused The receiving server is temporarily unable to process the message, possibly due to high load or maintenance. Fix: Your mail provider will usually retry. No action needed unless it persists. 450 – Mailbox unavailable The recipient mailbox exists but couldn’t accept the message at that time (often due to greylisting). Fix: Retry logic will often resolve this. If frequent, consider warming up your IP or authenticating your domain better. 451 – Requested action aborted: local error in processing The recipient’s mail server encountered a temporary internal error. Fix: Usually nothing is wrong with your message. Just wait and retry automatically. Diagnosing SMTP Errors in Practice To identify the source of a failed transactional email, follow these steps: Locate the error in your logs or bounce report. MailChannels logs and dashboards will show the full SMTP response, including the status code and any accompanying text. Check the recipient domain. See if the issue is isolated to Gmail, Yahoo, or another provider. Some ISPs have stricter policies. Look for patterns. Are multiple emails failing with the same error? That can indicate a misconfigured DNS record, blocked IP, or rate limit. Verify your domain authentication. Use tools like MailChannels Auth Checker to confirm SPF, DKIM, and DMARC are passing. Review your email content. Even transactional emails can trigger filters if they include attachments, shortened URLs, or aggressive language. Related Resources How to Monitor Email Performance (Delivery, Opens, Errors) SPF, DKIM, and DMARC for Transactional Email What to Do When Users Don’t Receive Their Transactional Emails How to Read an Email Header (with Real Examples) Take Control of Your Delivery Pipeline SMTP errors aren’t always your fault, but how you respond to them can make or break the email experience. Understanding 4xx and 5xx codes puts you in control of your delivery pipeline and helps you fix issues before users notice. Want built-in bounce intelligence and proactive deliverability management? Explore MailChannels Transactional Email and ensure your messages land where they’re supposed to.