Email bounce back: what it means and how to fix it
A bounce-back is an automatic message telling you your email was not delivered. It looks alarming, but it almost always explains the reason in one line.
Read the line that explains the reason and the code next to it. "Address not found" or code 5.1.1 means the address does not exist: check the spelling. A 5.7.x code means you were blocked by policy or failed authentication. A 4.x.x code is temporary, and the message is usually retried. Fix the cause, then send again.
Free tool: test your domain's records with the MX, SPF, DKIM and DMARC checker.
What the common bounce messages mean
Gmail: "Address not found"
From the Mail Delivery Subsystem: "Your message wasn't delivered to … because the address couldn't be found, or is unable to receive mail." This is almost always a 550 5.1.1: a typo or a closed account.
Outlook / Microsoft 365: "Delivery has failed to these recipients or groups"
The opening line of Microsoft's non-delivery report. Below it you will see the failed addresses, a plain reason, and "Diagnostic information for administrators" with the code. Common ones: 5.1.10 (recipient not found), 5.4.1 (recipient address rejected), 5.7.23 (SPF failed) and 5.7.509 (DMARC rejected).
"Returned mail" or "Undeliverable"
The same thing from other providers. Look for the code (three digits, then x.x.x).
How to fix each cause
| You see | Likely cause | Fix |
|---|---|---|
| Address not found, 5.1.1, 5.1.10, 5.4.1 | Typo or the mailbox was closed | Check spelling; ask for a current address |
| Mailbox full, 4.2.2 / 5.2.2 | Their inbox is over its limit | Try later or contact them another way |
| 5.7.1, "rejected by policy" | Their filter blocked you | Check your sender setup; avoid spammy content |
| 5.7.26, 5.7.23, 5.7.509 | SPF, DKIM or DMARC failed | Fix your domain's DNS records |
| Message too large | Attachment over the limit | Send a cloud link instead (Gmail and Outlook.com limits are 25 MB) |
| 4.x.x, "will retry" | Temporary problem | Wait; most systems retry automatically |
Getting lots of bounces from a list?
If a newsletter or outreach campaign produces many bounces, the list itself is the problem. Bounce rates over about 2% hurt your reputation, and above 5% you are in danger territory. Clean the list before the next send and remove every hard bounce.
Bounce-backs for emails you never sent
If you get bounce messages for emails you did not send, someone may be forging your address, which is called spoofing. Do not click links in those messages. If you own the domain, set up SPF, DKIM and DMARC so receiving servers can reject the forgeries. If it is a personal account, change your password and turn on 2-step verification in case your account was used.
Questions people ask
- Does a bounce back mean I am blocked?
- Only if the code says so (usually 5.7.x). Most bounces are simple typos or closed accounts.
- Did the person get my email if it bounced?
- No. A bounce means it was not delivered. A 4.x.x warning that says it will retry may still be delivered later.