Email Bounce Errors Explained — What the Error Codes Mean
When an email can't be delivered, the sender receives a bounce message (also called a Non-Delivery Report or NDR) containing an error code. This guide explains what the most common codes mean and what to do about them.
Most Common Bounce Errors
550 — User does not exist / No such user here
The email address you sent to doesn't exist. Either the address has a typo, the recipient's mailbox has been deleted, or the domain is no longer active. Double-check the address with the recipient.
550 — Message rejected as spam
The receiving server has identified your email as spam. This is usually caused by missing SPF or DKIM records, poor email content (too many links, all caps subject lines, spam trigger words) or your IP being on a blacklist. See our article: Why Are My Emails Going to Spam?
552 — Mailbox full / Over quota
The recipient's mailbox is full. You cannot send to them until they clear space. If this is your own mailbox, log into DirectAdmin and increase the quota or delete old emails.
421 / 450 — Try again later
A temporary error — the receiving server was busy or temporarily unavailable. Your mail server will automatically retry sending for up to 72 hours. If it still fails after that, you'll get a final bounce.
503 — Valid RCPT command must precede DATA
Your email client is not authenticating properly before sending. This is usually an SMTP authentication issue — make sure authentication is enabled in your email client settings and your username/password are correct.
551 — This mail server requires authentication
You are trying to send email through a Hostking mail server for a domain hosted on Hostking without authenticating first. Make sure SMTP authentication is enabled and you're using your full email address as the username.
554 — Message rejected
A catch-all rejection for various policy reasons — often a blacklist or content filter issue. Check mxtoolbox.com/blacklists to see if your domain or IP is listed.
Domain has exceeded max defers and failures per hour
Your domain has sent too many emails that bounced or were deferred. This usually means you have an infected script on your website sending spam, or a misconfigured email address generating thousands of bounce messages. Open a support ticket immediately if you see this — it can affect all email for your domain.
What to Do With a Bounce Message
- Copy the full bounce message text
- Check the error code in this article
- If it's a recipient address issue, contact the recipient via phone or another email
- If it's an authentication or spam issue, follow the linked troubleshooting articles
- If you can't resolve it, paste the full bounce message into a support ticket at my.hostking.host
Need Help?
Our support team is available every day from 7am to 10pm via live chat at hostking.host or by opening a ticket at my.hostking.host.
