Why Emails Bounce (and What the Bounce Message Is Telling You)
A bounce message is the receiving server telling you why it refused your email. It looks like gibberish, but it nearly always contains a short code and a sentence that name the exact problem. Once you can find those, most bounces take a minute to diagnose.
What a bounce is
When an email can’t be delivered, a mail server sends you an automatic reply. It usually comes from ‘Mail Delivery Subsystem’, ‘Postmaster’ or ‘Microsoft Outlook’, with a subject like ‘Undeliverable’ or ‘Delivery Status Notification (Failure)’.
A bounce is good news in one sense. You know the email didn’t arrive, and you have been told why. An email that silently lands in a spam folder gives you neither.
How to read a bounce message
Skip the friendly summary at the top and look for a line like this:
550 5.1.1 The email account that you tried to reach does not exist.It has three parts:
- The three-digit code (550). If it starts with 4, the problem is temporary and your server will keep trying. If it starts with 5, the problem is permanent and the email will not be delivered until something changes.
- The longer code (5.1.1). The middle number gives the area: 1 is the address, 2 is the mailbox, 4 is the connection between servers, 7 is security or policy.
- The sentence. This is written by the receiving server and is usually the most useful part. It often includes a web link to the provider’s explanation.
Common bounces and what to do
Swipe sideways to see the whole table.
| What the bounce says | What it usually means | What to do |
|---|---|---|
550 5.1.1 ‘User unknown’, ‘does not exist’, ‘recipient not found’ |
The address is wrong or the mailbox has been closed. | Check the spelling. If it is right, contact the person another way. |
5.1.2 or 5.4.4 ‘Domain not found’, ‘Host not found’ |
The part after the @ is misspelt, or that domain’s email set-up is broken. | Check the domain. If it is correct, the problem is on their side. |
552 5.2.2 or 452 4.2.2 ‘Mailbox full’, ‘Over quota’ |
The recipient has run out of storage. | Tell them by phone or another address. Resend once they have cleared space. |
552 5.3.4 ‘Message too large’, ‘Size limit exceeded’ |
The email and its attachments are over the recipient’s limit, often 20 to 25MB. | Send a link to the file instead. Attachments grow by about a third in transit. |
421 or 451 4.7.0 ‘Try again later’, ‘Temporarily deferred’ |
The receiving server is busy, or is slowing you down because it doesn’t yet trust you. | Usually nothing. Your server retries. If it happens every time, check your sending reputation. |
4.4.7 ‘Delivery time expired’, ‘Retry timeout exceeded’ |
Your server tried for several days and gave up. | The recipient’s server is down or refusing connections. Try another route to reach them. |
550 5.7.26 (Gmail), 550 5.7.515 (Outlook.com), 5.7.23, 5.7.509 |
Your email failed its SPF, DKIM or DMARC checks. | Fix your domain’s email authentication. See below. |
550 5.7.30 ‘Basic authentication is not supported for Client Submission’ |
A scanner, website form or app tried to send through Microsoft 365 with a username and password. Microsoft switches this off by default at the end of December 2026. | Switch it to Microsoft sign-in or another way of sending. See my 5.7.30 page. |
554 5.7.1 ‘Blocked using’, ‘Listed on’, with a blocklist name |
Your sending server or domain is on a blocklist. | See my blacklist check and removal guide. |
550 5.7.1 ‘Unknown sender’, ‘Sender address rejected’, 501 5.1.7 ‘Bad sender address’ |
You are sending from an address your account isn’t allowed to use. | Add the address as an alias or mailbox on your account. See my aliases guide. |
550 5.7.1 ‘Relay access denied’, ‘Relaying denied’ |
Your email app isn’t logging in to the outgoing server properly. | Check the outgoing (SMTP) server, username and password in your email app. |
550 5.4.1 ‘Recipient address rejected: Access denied’ |
A Microsoft 365 recipient’s system doesn’t recognise the address. | Check the spelling. If customers get this when emailing you, your own set-up needs checking. |
5.1.8 ‘Bad outbound sender’, 5.7.705 or 5.7.708 |
Microsoft has blocked your own account or organisation for sending spam. | Treat it as a possible hacked account. See my hacked email guide. |
The code 550 5.7.1 appears several times because it is a general ‘refused by policy’ code. Always read the sentence that follows it.
Authentication bounces: the ones that are your side to fix
Since 2024, Google, Yahoo and Microsoft have tightened their rules on who they accept email from. Email that fails SPF, DKIM or DMARC used to go to spam. More and more often it is now refused outright.
If a bounce mentions ‘unauthenticated’, ‘SPF’, ‘DKIM’, ‘DMARC’ or ‘does not meet the required authentication level’, the fix is in your domain’s DNS records, not in the email itself. Rewriting the message will not help.
Start with Check your email is set up correctly, then read Google, Yahoo and Microsoft email rules for what each provider expects.
Bounces for emails you never sent
If your inbox fills with bounces for emails you don’t recognise, check your Sent folder first.
- The emails are not in Sent. Someone is faking your address on spam sent from elsewhere. Your account has probably not been broken into. A DMARC policy of quarantine or reject cuts this down sharply.
- The emails are in Sent, or your password has stopped working. Assume your account has been hacked and follow my hacked email guide now.
When there is no bounce at all
No bounce does not mean the email arrived in the inbox. It means a server accepted it. It may still be in a spam folder or held in a quarantine the recipient never checks.
If a customer says they didn’t get your email and you have no bounce, read ‘It sent’ doesn’t mean ‘it arrived’.
When people emailing you get the bounce
If customers tell you their emails to you are bouncing, ask them to forward the bounce message. The same codes apply, pointing at your set-up this time.
The usual causes are a full mailbox, an address that was never recreated after a move to a new provider, or DNS records still pointing at an old email host. My guide to switching email provider covers the last two.
What to keep when you ask for help
Keep the whole bounce message, not a screenshot of the first few lines. The useful details are at the bottom: the code, the sentence, the name of the server that refused the email and the time.
Need help fixing this?
I provide:
- Free initial assessment
- Clear explanation of the issue
- Fixed-price solutions
- UK-based support
Forward me the bounce message and I’ll tell you what it means and whether the fix is on your side or theirs.