You sent an email and shortly after you get a message back: "Your email was not delivered", "Undelivered Mail Returned to Sender" or "Mail Delivery System". A notice like this is called a bounce. It means your email did not reach the recipient. The notice tells you why. In this article you'll learn where to find that reason, what the most common error messages mean and what to do in each situation.
Where is the reason?
A bounce notice has three parts:
- At the top a short explanation in plain language.
- Below that the email address of each recipient and the reason the recipient's mail server gave. This is the part that matters most.
- At the bottom or as an attachment a technical report and your original message.
The reason looks technical, for example:
<[email protected]>: host mx.example.com said: 550 5.1.1 User unknown
You don't need to understand all of it. Look at two things: the words (here "User unknown") and the code (here 550 or 5.1.1).
- If the code starts with a 5, it's permanent. The email will not be retried.
- If the code starts with a 4, it's temporary. The mail server will try again later by itself.
Below you'll find the most common reasons. Look up the words from your notice in the list.
1. The email address doesn't exist
You'll recognise it by: User unknown, does not exist, no such user, mailbox unavailable, recipient address rejected, invalid recipient or code 5.1.1.
The recipient's domain exists, but the address in front of it doesn't. Usually there's a typo (jhon@ instead of john@), or the recipient no longer uses this address, for example after changing jobs.
What to do: check the address letter by letter. Pay special attention to dots, hyphens and swapped letters. If you're sure the address is right, ask the recipient another way whether it's still in use.
2. The domain doesn't exist
You'll recognise it by: Host or domain name not found, Domain not found, no MX record, NXDOMAIN or code 5.1.2.
The part after the @ is wrong. Often it's a typo like gmial.com or hotmial.com. Sometimes the recipient's domain has expired or its email settings are not set up correctly.
What to do: check the part after the @. If it's correct, let the recipient know another way that their email isn't working. They'll need to fix it with their own provider.
3. The recipient's mailbox is full
You'll recognise it by: mailbox full, quota exceeded, over quota, insufficient storage or code 5.2.2 (or 4.2.2).
The address exists, but the recipient has no room left for new email.
What to do: try again in a few hours or a day. If it's urgent, let the recipient know another way that their mailbox is full. Only they can free up space.
4. Your message is too large
You'll recognise it by: message too large, message size exceeds, size limit or code 5.3.4 (sometimes 5.2.3).
Your message is larger than the recipient accepts. This is almost always caused by attachments: photos, videos or large PDFs. Note that an attachment grows by about a third when it's sent, compared to the file on your computer.
What to do: spread the attachments over several emails, resize photos before attaching them, or put large files in an online storage service and send a download link instead.
5. The recipient's server refuses our server
You'll recognise it by: refused to talk to me, blocked, blacklisted, listed at, not allowed, access denied, IP followed by a number, or code 554 or 5.7.1.
This isn't about your message or your address, but about the server it was sent from. Some providers keep lists of servers they don't trust or don't know yet, and refuse everything that comes from them. Large providers in particular can be strict about this.
What to do: there's nothing you can do about this yourself. Reply to the bounce notice or contact us and include the notice. We'll get in touch with the recipient's provider and make sure our server is accepted again. If it's urgent, contact the recipient another way in the meantime.
6. Your message is seen as spam
You'll recognise it by: spam, message rejected, content rejected, suspicious, virus, policy or code 5.7.1, without anything about an IP address or a block.
The recipient has assessed the content of your message and doesn't trust it. This happens, for example, with attachments that are often misused (such as .zip or .exe files), with many links, or with text that looks like advertising.
What to do: send the message again without the suspicious attachment or with fewer links. Put files in an online storage service and send a link instead. If it happens with many different recipients, also read Why isn't my email arriving? (in Dutch).
7. Your domain isn't set up correctly for email
You'll recognise it by: SPF, DKIM, DMARC, unauthenticated, not authorized to send, sender verify failed or code 5.7.23, 5.7.25, 5.7.26 or 5.7.27.
The recipient can't verify that the email really comes from your domain. This happens when your domain's DNS settings for email are missing or wrong, for example after moving your domain or website.
What to do: check your domain's SPF, DKIM and DMARC settings. You can read how in Setting up SPF, DKIM and DMARC (in Dutch). If you can't figure it out, contact us.
8. Temporary problem: the email is delayed
You'll recognise it by: try again later, temporarily, greylisted, deferred, delayed, "THIS IS A WARNING ONLY" or a code starting with a 4.
The recipient isn't accepting your email right now, for example because of heavy traffic or an outage. The message hasn't failed yet.
What to do: nothing. Our server automatically keeps trying for several days. If it still doesn't work in that time, you'll get a separate notice that it has failed for good.
9. An error in the recipient's email settings
You'll recognise it by: loops back to myself, mail loop, too many hops, relay access denied or no route to host.
The email settings of the recipient's domain point to the wrong place, so your email can't be delivered anywhere.
What to do: first check the address for typos. If it's correct, let the recipient know another way that their email isn't working. This has to be fixed on the recipient's side.
Bounce notices for email you didn't send
Are you getting bounce notices for messages you never sent? Then someone else has used your address as the sender of spam. Unfortunately, they can do that without having access to your mailbox. The recipients refuse the spam and send the notice to the address that was on it: yours.
What to do: look in your Sent folder. If the messages aren't there, you can ignore and delete the notices. If they are there, or you notice anything else unusual, change your email account's password right away and contact us.
When should you contact us?
- The reason is reason 5: the recipient's server refuses our server.
- It happens with several recipients at once, with addresses you're sure are correct.
- You don't know which reason it is.
Reply to the bounce notice, or contact us and include the notice. With the reason and the address in it, we can look into it quickly.
Frequently asked questions
Did the recipient get my email anyway?
No. With a permanent bounce (a code starting with 5) the email did not arrive. If you want the recipient to get it, you'll need to send it again once the problem is fixed.
Do I need to send my message again?
With a permanent bounce, yes, after you've fixed the cause, such as a typo in the address. With a delay (a code starting with 4), no: our server tries again by itself.
Why does it only happen with one recipient?
Then it's almost always down to that one address or the recipient's provider: a typo, a full mailbox or a provider that refuses our server. Read the reason in the notice and look it up in this article.
I don't understand the error message. What now?
Reply to the bounce notice or contact us and include the notice. We'll tell you what's going on and what you can do.