When an email fails to deliver, you'll usually receive a bounce-back message (also called a Non-Delivery Report or NDR) containing an error code and description. This guide explains the most common error messages and what to do about them.
When You Would Use This Guide
Refer to this guide when you receive a bounce-back email after sending a message, or when investigating email delivery issues using the Track Delivery tool in cPanel.
Please note: Screens and options may vary slightly depending on your cPanel version and hosting plan.
Understanding Error Codes
Email errors use a three-digit code system. The first digit tells you the general category:
- 4xx (Temporary) — The server encountered a temporary problem. The sending server will usually retry automatically. No action is needed unless the retries keep failing.
- 5xx (Permanent) — The delivery has permanently failed. The email will not be retried. You need to fix the issue and resend.
Common Error Messages
550 No Such User / Mailbox Not Found
What it means: The email address you sent to doesn't exist on the receiving server. What to do: Check the recipient's email address for typos. Confirm the address is correct with the recipient. If it's an address on your own server, create the email account in cPanel.
550 Mailbox Full / Over Quota
What it means: The recipient's mailbox has reached its storage limit and can't accept new messages. What to do: If it's your own mailbox, free up space by deleting old emails or increasing the quota in cPanel. If it's someone else's mailbox, let them know and try again later.
550 Message Rejected / Blocked by Policy
What it means: The receiving server has rejected your email based on its security policies. This can happen due to failed SPF/DKIM checks, blacklisted IP addresses, or content that triggers spam filters. What to do: Check your SPF, DKIM, and DMARC records using cPanel's Email Deliverability tool. Check if your server's IP is on any blacklists using a tool like mxtoolbox.com. Review your email content for spam-like characteristics.
553 Authentication Required / Relay Denied
What it means: Your email client is trying to send email without authenticating (logging in) first, or the server doesn't recognise you as an authorised sender. What to do: Check your email client's outgoing (SMTP) server settings. Ensure "Require authentication" is enabled and your username (full email address) and password are correct.
421 Too Many Connections / Rate Limited
What it means: You've sent too many emails in a short period and the receiving server is temporarily limiting your connections. What to do: Wait and try again later. If you're sending bulk email, stagger your sending. Consider using a dedicated email sending service for large volumes.
451 Temporary Service Unavailable
What it means: The receiving server is temporarily unavailable (maintenance, overload, or technical issue). What to do: Wait — your server will automatically retry delivery. If the email is still failing after 24–48 hours, contact the recipient through an alternative method.
554 Transaction Failed / Message Refused
What it means: A general rejection. Often caused by the message being identified as spam, containing a virus, or having an attachment type that's blocked. What to do: Review your email content. Remove or rename suspicious attachments. Check that your sending server isn't compromised or blacklisted.
530 Authentication Required / STARTTLS Required
What it means: The receiving server requires an encrypted (TLS) connection that your server isn't providing, or authentication is needed before sending. What to do: Ensure your email client uses SSL/TLS or STARTTLS for the outgoing server. Check that port 465 (SSL) or 587 (STARTTLS) is configured.
Connection Timed Out
What it means: The sending server couldn't establish a connection to the receiving server within the allowed time. What to do: This is usually a temporary network issue. Wait for automatic retry. If persistent, check that the recipient's MX records are correct and their mail server is online.
How to Find the Full Error Message
From a bounce-back email: The bounce message usually contains the error code and a description from the receiving server. Look for lines starting with "Remote host said" or "Diagnostic-Code."
From cPanel's Track Delivery tool:
- In cPanel, go to Track Delivery in the Email section.
- Search for the recipient's address.
- The result column shows the full delivery status and error message.
Tips
- Most temporary (4xx) errors resolve themselves within a few hours as the sending server retries.
- For persistent delivery problems, the Email Deliverability tool in cPanel is your best starting point — it checks SPF, DKIM, and DMARC configuration.
- If your server's IP address is blacklisted, contact your hosting provider. They may be able to request delisting or move you to a clean IP.
- Keep your email accounts secure — compromised accounts sending spam will quickly get your server blacklisted.
