Mail Delivery Failed: Returning Message to Sender

Getting an email returned with a “mail delivery failed” subject line usually means the recipient’s mail server refused it. The good news is that the returned message explains why. This guide walks through how to read it, what the most common reasons are, and what you can do about each one without needing a technical background.

Your email did not disappear. A mail server somewhere between you and your recipient turned it away and sent it back with a note attached. That note is the returned message sitting in your inbox, and it names the address that failed along with the reason the other server gave.

What the Returned Message Is

Mail servers send these notices automatically. You will see the sender listed as something like “Mail Delivery System” or “mailer-daemon,” which is not a person and cannot be replied to. It is your mail server reporting back to you.

Three things inside the message are worth finding:

• The failed address. If you emailed several people, only the addresses listed here were rejected. Everyone else received it.
• The reason. Usually a short line of text such as “No such user here” or “mailbox full.” This came from the recipient’s server, not yours.
• A copy of what you sent. Helpful when a website form or plugin sent the email rather than you.

If the returned message looks like a wall of technical text, paste it into the Bounceback Parser Tool and it will pull out the important parts for you.

The Number That Tells You Whether to Try Again

Somewhere in the returned message you will see a three digit number. You only need to check the first digit.

If it starts with 5, the failure is permanent. Sending the same message to that address again will produce the same result. These are called hard bounces.

If it starts with 4, the problem is temporary. Your server keeps the message and quietly tries again for a few days before giving up. These are soft bounces, and you often do not need to do anything at all.

You may also see a longer code like 5.1.1 next to it. That is a more specific version of the same information. The three part format defined in RFC 3463 tells you whether delivery failed, where the problem came from, and what the exact condition was.

What the Most Common Errors Mean

What you see	What it means	What to do
550 5.1.1, “no such user”	That mailbox does not exist	Check the spelling, or confirm the person still works there
550 5.1.2, “domain not found”	The part after the @ symbol is wrong or the domain is gone	Verify the domain name with the recipient
552 5.2.2, “mailbox full”	Their inbox is out of space	Contact them another way and try again later
550 5.7.1, “message rejected”	Their server blocked it on purpose	See the authentication and reputation section below
552 5.3.4, “message too big”	Your attachment exceeded their limit	Send a download link instead of the file
421, “service unavailable”	Their server was busy or throttling	Wait. Your server will retry on its own

Why Emails Get Returned

The address is wrong or no longer active. One misplaced letter is enough. So is emailing someone who left their company months ago.

Your mail is going to the wrong place. This one catches a lot of people. If your domain uses an outside email provider such as Google Workspace or Microsoft 365, but your hosting account still thinks it handles your mail, your server looks for a mailbox it does not have and returns “no such user here.” The Email Routing setting in cPanel controls this and needs to be set to Remote Mail Exchanger.

Your domain is missing its email verification records. SPF, DKIM, and DMARC are DNS records that prove your mail is really coming from you. Without them, large providers increasingly refuse the message outright. Google and Yahoo started requiring these records from high volume senders in February 2024, and mail that does not meet the requirements now gets rejected rather than just filtered.

Something on your website is sending the email. Contact forms, WordPress comment notifications, and order confirmations all send mail automatically. If the notification address inside those settings is outdated, every trigger creates another bounce. Repeated comment spam is a frequent cause, and cleaning up WordPress comment spam usually stops it.

Your server’s reputation is the problem. If the IP address your mail leaves from has been flagged for spam, receiving servers may reject everything from it.

How to Check What Happened in cPanel

1. Log in to cPanel and open Track Delivery in the Email section. Click Run Report. You will see a list of recent messages with a green check, a red X, or a warning icon next to each one.
2. Enter the recipient’s address in the optional filter box to find one specific message and see the exact response their server gave.
3. Open Email Deliverability and look at the status column for your domain. If it shows a problem, click Repair and cPanel will add the missing records for you. Give DNS changes a few hours to take effect.
4. Check Email Routing if your domain’s email is hosted somewhere other than your hosting account.

What You Can Fix Yourself

Correcting a typo, updating a notification address in WordPress, compressing an attachment, or clicking Repair in Email Deliverability are all safe to do on your own. So is waiting out a temporary failure.

When to Ask Support for Help

Anything involving the server itself is worth handing off. That includes blocklist removal requests, reverse DNS records on a VPS or dedicated server, mail server configuration, and reviewing mail logs. Our support team is available 24 hours a day, and forwarding the full returned message speeds things up considerably.

Sending more email as you grow? Accounts on shared servers share a sending reputation with the other sites on that machine. A Managed VPS gives you a dedicated IP address and control over the records that affect delivery, with our team handling the setup.